So this is my problem:
I use testingcheats a lot (they are unstable i know) but this problem has caught my attention.
I used the option "Rent Furnished Apartment" from these cheats, and my sim is now getting bills (I assume from the extra value of the furniture.)
How can I stop these bills? They end up making the apartment cost a lot more than it should.
My game is completely patched since it's UC, and there is no glitches on my hardware.

The testing cheats are not unstable. Having them on all the time is only dangerous if you do silly things with them. Renting a furnished apartment is not a silly thing to do with them at all.

What do you mean, the sim is getting bills? Of course he's getting bills - they come to apartments on Tuesday and Thursday just like they do to other residences, plus they pay the landlord at the mailbox every Monday.

If bills arrive on any other day, something is wrong, but again it has nothing to do with the testingcheats. The most likely culprit is a mod, possibly one that was created prior to AL that is now acting unpredictably with the apartment code, or possibly two mods interacting in unpredictable ways. So next time bills arrive on an unexpected day (make sure it is a bill and not coupons or a love note), quit without saving, back up your game, and remove your Downloads folder to a safe location. If the bill still comes, then I'm wrong and the problem's not in your Downloads folder. If it doesn't, then all you have to do is test till you find the culprit(s).

See the Game Help FAQ you were directed to before you were allowed to start this thread for a description of the most efficient method, popularly called the 50/50, of testing the downloads folder. It's tedious, but it won't take you as long as you think it will.

Ah! I see, I did not know the payments were required with the rent. Thanks!