Ansjovis

For me personally, it'd be mushrooms. I really don't like them haha.
But generally, I guess roe/caviar on pizza is pretty bad.

I recently saw a picture of pizza with kiwis on it, so that.

Spaghetti, I tried that combination yesterday for the first and last time.

Any insect. I know that if it exists, someone somewhere likes to eat it, but I don't want any delicacy cockroaches on my pizza.
(On the flip side of things: I think more pizzas should add sweet corn. It's soooooo good but only the vegetarian pizzas have it for some reason. Also those restaurants who claim to be "Italian traditional" or whatever should stop being so stingy with ketchup. >:( )

Potato, pasta, cabbage, bagel, leather and aluminium.

cyanide.

Human meat I guess?
On a more serious side, I've tried one that had cooked beans on it, and not really a fan of the texture. It's a pity, pizza + chili con carne sounds great :P

Fish

Surströmming, you'd never be able to use that oven again.

Fish and pineapple :s

nuclear waste