How to replace stripping nuts?

I got a nut stripped on the stud last time when I flush the cooling system. Now I cannot make it tight enough and cannot back it off. I am worrying next time when I change the coolant or thermostat how I can take it off, although it doesn't leak now. Any advice? Thank you!

The nut is one of the two nuts which connect a coolant hose to the thermostat housing on the 2000 camry(4 cylinders). So I don't have big space to work on.

You will probably need to find a nut splitter to break the nut off and have the stud rethreaded with a die.