If a loan is paid automatically each month through direct debit, will the direct debit stop when the loan reaches zero or will money continue to be withdrawn until the person paying the loan stops the direct debit?
The direct debit is an authority to withdraw money from your account. Depending on how it was set up it may limit how much may be withdrawn, how often and for how long. I haven't seen a direct debit agreement that limits the total that may be withdrawn, but they may exist.
A reputable creditor will stop taking your money when they're supposed to, which would be when the loan is paid off.