Fuzakeru ふざける (巫山戯る): to romp; to gambol; to frolic; to joke; to make fun of; to flirt; to mess around; to screw around; to josh; (taken from EDICT)
Fuzakenaide ふざけないで is the negative request form of the verb, asking that the listener to desist from messing around. Sticking ~yo on the end gives it emphasis.
'zakenna is a slangy drawled version of the above. To sound tougher and scarier add ~zo instead of ~yo, though you might need a shiny suit and a tight perm to pull it off.
NB: This is considered very rude in all its forms and, though it sounds fairly innocuous when translated literally into English, would be taken as a 'Fuck you!" or "Stop fucking about!" by a native.
mou yame nasai. kyou ha yakeni fuzakete iru'n da kara
Quit already! 'cos today you're just pissing about.