Cancel your plan.
Takes about a minute. The weeks you have already paid for still run out normally — cancelling stops future charges, it does not cut anything short.
We text a code because a phone number typed into a web page is not proof it is yours — anybody who knew it could otherwise cancel your plan.
Prefer to do it by text? Text CANCEL PLAN to Carl's number from the handset your plan is on. That needs no code, because the text is the proof. (CANCEL on its own is a carrier keyword that only stops text messages — it will not cancel your plan.)
The purchase terms set out what happens to minutes you have already paid for.