Take Petra's advice.
Keep this posted on your wall. "DO NOT - EVER NEVER - WORK ON HOURLY JOBS THAT YOU CAN'T FINISH IN TIME ALLOCATED. UNLESS YOU HAVE RELATIONSHIP GOING WITH THE CLIENT, DO NOT TRUST THEIR STORY OR EXCUSES"
Always make sure that the payment for hours are verified. Always use Upwork's time tracker with all the optional and required annotation - no manual time even if client says so.
Ask the client whether he would like you to stop working and leave the remaining tasks for the next week when you reach the limit, or whether he would like to raise the limit so that you can continue with the tasks. Present those clearly as the only two options and let him choose.