If you're only half-serious about it, I'd rather you didn't come.
Literal
Fun-half [conditional-なら] come-not want.
面白半分 means 'half out of curiosity or amusement' — doing something as entertainment rather than seriously. ~なら sets a topical conditional: 'if that's your attitude.' ~ないで欲しい is a direct way to say 'I want you not to do X,' more pointed than ~ないでください.