Flex spending account (FSA)?

So i have little under $600 left on my FSA account. I want to pay for my nephew bill which is 5 something and i know if i pay for it they will eventually deactivate my card. so my question is when they do deactivate it will i have to pay them back right away or can i just not use it anymore and let them keep taking the $600 i have left to pay for it.