1. Fix your drivetrain - seems like every time you were in a position to go around them or outrun them your chain fell off or you were "nursing" your drivetrain.
2. Hook up a Go-Pro or equivalent and keep riding. If they're there the next time, film them and go to the cops, if they're not, just carry on.
3. As a last resort, and assuming that this is a repeated issue and you don't get any satisfaction from law enforcement, consider Mace (never a gun), but ensure that you're on safe legal ground wrt carrying Mace in your state/jurisdiction, and that the conditions under which you use it are justified self-defense in response to a clear and recorded assault (IANAL, so take this "legal advice" for what it cost you)