How would you homebrew an oathbreaker Warlock?

I have a warlock player who's been taking action pretty much directly against their patron, and failing to follow the tasks their patron has set out for them. Things haven't reached a tipping point yet, and the warning signs will be growing more and more clear to give the player a chance to get back on the straight and narrow (relatively speaking), but if they choose not to follow these warnings it seems as though it would make sense for the patron to withdraw support.
However, I don't just want to remove all warlock abilities, because that would obviously completely cripple the character, leaving them with only rudimentary combat skills and a little more health than a level 0 character. Ordinarily this would be a great incentive to, y'know, get them to not abandon the patron, but it's a new player and the goal of my group is to get new folks into P&P games, so that's really not an option. Some power loss is of course fine, to provide some consequences, but how would you homebrew an oathbreaker warlock?
Thanks! EDIT: Some missing context relevant to a couple of replies; the player started as a bard, they switched to warlock because obtaining the pact happened in-game. Basically, they freed one of a set of nine old gods, and they made a deal with said god to free the other eight in exchange for power. The player has so far failed to make any steps in that direction. EDIT 2: Upon further consideration, I'll just have the god offer further incentive for following directions rather than whipping away all the power. Thanks all!