You authorized the ACH! Read the fine print in your payday loan contract; it's standard procedure.
Rather than attempt to get the full amount you owe, the payday loan company will submit smaller amounts for ACH debits. Typically they review your account, take note of the day you are typically paid by your employer, and submit the ACH the night before your funds are deposited into your checking acct by your employer or by you at the bank.
Additionally, payday loan companies can access your bank acct status via, Accurint, Lexus-Nexus, DP Bureau, etc.
Finally, if they choose to make your life more miserable, they will place you in SCAN (Shared Check Authorization Network).
You need to close your bank account ASAP. You can open another in the same bank if you prefer. Just make certain you are assigned a new bank acct number.
Next, contact the payday loan company and make arrangements to pay them a small monthly fee until you pay back the principal.
You do owe them the $$ so honor your agreement.
If they take you to small claims, be happy. Just SHOW UP. There is nothing to fear. The "judge" may throw it out (they HATE payday loan companies) or they will get a judgment and you can make SMALL monthly payments.
You cannot go to jail!