Just force feed him a burnt pizza. He'll choke on it.

Play "I wanna be the guy" And learn from that game how to kill Mario. This is what I have in mind. Just after the gates of the last castle, have a room filled with 1-up mushrooms. Enough for 99 lives. Don't worry, He'll use them all up in no-time, this is to increase the pleasure. Then, there is a checkpoint after that room. The room looks simple enough. Just a few statues in the background. But when mario walks past it *WHAM*, it crushes him. After that, there is a gap filled with lava. There are some fake platforms, most are holagrams. He'll most likely fall through them, if he can get through that, then there is a corridor with lazers that apprear....
you know what, Browser isn't smart enought for that. Just make one of the big castles rigged with a bomb, so when he meets toad and when he's saying "Thank You", there is a beeping sound. *beep beep beep beep beepbeepbeepbeepbeepbeepbeepbeepbeepbeep* What the fu- *BOOOM!*

ovisly with a giant never ending sea of lava

Have him fight two bosses at once.

Send a giant wall of fire to attack him in his sleep. Or poison his mushrooms.

Put Mario in a room with no power-ups. B4 u put him in that room, make a cloning machine to make Bowser-Clones that will obey your every command. Then, your clones jump through holes in the walls and burn Mario to a crisp. If he jumps,don't worry. U will jump through a different hidden wall and finish him off with a 1-Hit-KO punch. If he used a back-up Mushroom, put him in the ground. In other words, punch downwards. He'll be stuck and then finish him off with a devastating inferno of flames. With your clones, of course.

Here's a good hint, stop making every trap easy for him to escape you should make more traps so that it would be impossible for Mario escape.

Erm... You made your doomsady device. Did you manage to find a way to kill Mario yet?

Here's one way: trap him in a room full of exploding coins. He won't know what hit him.

Yeah, or just lock all the doors and burn the paintings. He will die of starvation eventually.

Well, I have a few suggestions. But given that your doomsday device was made over a month ago, he probably already screwed that up for you. Anyway, these are tweaks to what you have already done. This involves getting a time machine. 

First, go back in time to Super Mario Bros. Take out all the flagpoles at the end of the levels. Then he can't finish the level, and time will just run out.
 
For backup, get rid of those ridiculous axes behind you on the bridge over lava. What in the heck were you thinking? "Oh, jumping on my head hurts Mario. His fireballs do not hurt me. How about I put this ax behind me next to the ropes holding the bridge over a POOL OF FREAKING LAVA!" 

Next, time travel to Super Mario Bros 3 (No point in going to Super Mario Bros 2, as that was just a dream anyway). Take out all of those stars in World 7 - Stage 7. 

Just to be on the safe side, take out all of those breakable bricks that you end up shattering when you land on them when Mario fights you. Unless they have the P Wing, he can't reach you. 

If he has a P Wing when he reaches you, make sure you told yourself to keep the key in your shell in case Mario finds a way to push you down the pit. That way that door to the Princess doesn't open when you fall and Mario will have to try to fall down the pit to get the key (which means he dies. We already know you survived it, but he can't)

Then, time travel over to Super Mario World. Tell yourself to not throw those enemies from your floating clown so Mario has nothing to throw at you. 

Finally, take that time machine back to your time, and Mario should be gone. Although, I must warn you that if you do this, Mario won't be around to join forces with you to get your castle back from Smithy. Let's be honest, you will not beat him on your own. So you gotta ask yourself if it's worth it.

I don't see bowser anywhere! He could just die on his own by eating a poison pizza he use to eat that and spaghetti a lot in the old cartoon show ahha

By the way they need another cartoon show it ended all the way to super mario world now there's more friends and enemies available make the wario bros useful for once

lure him into a room filled with nothing but lava...no platforms no koopas just lava

Upgrade your enemies and castle with guns, missles, lasers, and grenades, INVISIBLE grenades! Hahahahahahaha!
Also add wall barriers of death on top of the enemy that stretches infinitly!
Steal all of mario's important stuff, too: Coins, mushrooms, even 1ups!

Why not just put Mario up with a "job" to take care of his Chain Chomps and tell him that he is leaving on vacation? Then when Mario cleans the Chain Chomps' cage and the Chain Chomps will savagely attack him and a Lakitu will be there to record his demise. MWAHAHAHAHAHAHAHAHAHAHAHAHAHAHA! AHAHAHAHAHAHAHAHAHAHAHAHAHAHAHA!

Go out to the mushroom kingdom and take EVERY powerup that there is so Mario is stuck at small size for the entire game, which means 1 hit = dead. Then you need to put up barriers to block the end of the bridge in your castle where you stand so Mario can't jump over you or run under you. Then its impossible to win.

You could switch all of the mushrooms out and replace them with Poison Mushrooms