All colloquial:
"Nee"/"Naa" (depending on where you are locally, the sound can differ between those two. Both will be understood, though. Also possible and often heard: "Ne ne", two times the same sound)
"Nö" (More like: "Nööö". Only Nö would be too short, the person you're talking to would feel like you wan't to shrug them off. You should either stretch the "ö" or follow it by something else, like "Nö, habe keine Lust", "Nö, lieber nicht")
"Ach, lass' mal" ("Ach" beeing just a sound with no real function here, you could leave it out. This sentence would mean something like "don't bother (asking me again/talking about it)". Even less formal: "ach, lass' stecken", meaning bascially the same)
"Nope" (Okay, we're getting really into yoof-slang/yout language here, but you can hear that sometimes around those groups)
"Lieber nicht" ("I'd prefer not to")
There would probably be a dozen more, but they're all close or similar.
***source: Duolingo