you can count me out!

You can count me out! {informal} — Я на это не подписываюсь; Нет, это мне не подходит; Увольте!

Other ways of saying you'd rather not do

Well, I'd rather not, if you don't mind.
I don't really feel it's my kind of thing.
It seems a/an (adj. eg: boring, silly) sort of thing to do, if you ask me.
It's really not my cup of tea.
That's not exactly what I had in mind.
Actually, I don't really think I could find the time.

See also
[это не ко мне]
[сердце не лежит]