I Did Something I Regret

everything can be fixed. but sometimes the damage repair is more than the cost of the device. you can't possibly damage an electronic device without suspicion if there's a forensic analysis. but hardly anyone would do that because it's just way too expensive.
it really depends on the phone model you have.
basically the easiest way is to drill the pcb in the cpu chip. it will no longer work and unless someone opens it up no one will notice.
you can also fry it by inputting an electrical current bigger than it was supposed to. it will fry some circuits and it will no longer work and it can be a pain to find the cause. but be careful with that because it's a fire hazard. just wear rubber gloves and take out the battery.
if you want the information on the phone to be impossible to recover just damange the emmc chip. the data is stored there and if the chip is damage it can't be removed. just crack the chip. it's enough to damage the crystal oscillator.
you should had provided more info. phone model is important.

Wha?

Are you trying to ruin it so that you don’t have to pay the bill? That seems ass backwards and immature. Just tell them so they can have the service transferred. No need to ruin a perfectly good phone.

Water then let it dry.
What model?

uh, why?

?

You don't. You realize that you did something stupid and you'll think next time before doing it.