I'm going to forget to flush
Why do you want to do that?
I want their bathroom to smell like shit.
It's really bad form to not flush a toilet. Think about is someone did that in your house. Would you like that?
I don't care. Their place should be as smelly as possible. This is revenge.
Taking "revenge" by not flushing the toilet is silly. It just makes you look like the asshole here.