Polite option 1: This band isn't going in the direction I signed up for, and this is not what I want to do. Bye.
Polite option 2: Discussion with all of the band. Discuss whether you want to play covers (to entertain) or original songs (to 'educate' audiences). The second is generally harder to get gigs.
Not so polite option 3: You're a deluded bunch of losers with no talent and an appalling case of the Dunning-Kruger effect, and I'd rather sit in a darkened room sticking rock splinters into my eyes than carry on with this representation of the ninth circle of hell. See ya!
(Only wrap it up a little nicer, because you don't know how the future will pan out.)
It's all Danny Kruger's fault..