The thing is "Paladins lose their powers for an evil act" hasn't been part of mainline D&D's rules since 2008 and has never been a part of the most popular version of D&D in history.
You are either not playing 5e or are dumpster diving through previous editions to pull out a bad rule from them and insert it in as a house rule.
Yeah, 5e Paladins don't lose their powers for an evil act. However, according to the 5e rules (2013!PHB pg 86, "Breaking Your Oath") we do have:
"If a paladin willfully violates his or her oath and shows no sign of repentance, the consequences can be more serious. At the DM's discretion, an impenitent paladin might be forced to abandon this class and adopt another, or perhaps to take the Oathbreaker paladin option that appears in the Dungeon Master's Guide."
So in 5e it's not evil, it's breaking the tenets of their chosen oath. And it very much is listed as part of the 5e.
Mind you, this is also the most severe. A paladin breaking their oath who is penitent has a whole range of minor things, like holding prayer vigil, fasting, getting absolution from a cleric/paladin of their faith. So it is entirely within the player's ability to avoid that last bit, even after breaking one's oath willfully. And it doesn't come into play unless the oathbreaking was willful. "Ooops, you had no way to know it but..." doesn't break it.
EDIT:
@TwoSix , just saw in a later post that you mentioned that you don't feel that removing a character's power is ever an appropriate consequence. Now, neither Warlock nor Cleric have any rules about removing power. The paladin bit above does, but replacing it with either another class or the oathbreaker, and is entirely within the player's control not to let escalate to that point, so seems like it is a conscious choice of the player to swap abilities. Is that acceptable?