Brownie point for trying :)
You shouldn't have done that - Das hättest Du nicht tun sollen.
You should have left (it) well alone (BE) - Das hättest Du lieber bleiben lassen
You should have left (it) well enough alone (AE)

And if the context is that you pursued something beyond a given state and thus ruined it, you could say: You should have left it at that.
HTH

Actually, "you shouldn't have done that" is just fine, but "you should have left well enough alone" is good too.

Thanks a lot everyone, much appreciated. :-)