What can kill a god?
Q seemed pretty afraid of Guinan, and she certainly gave the impression that she could do him some damage, even with his powers.
It might have been just fear for the chaos it would cause with "lesser" species, but Q was adament that his son follow the Continuum's rule about not provoking the Borg, which could indicate some fear.
During the Q Civil War, their fatal "shots" at each other in the Continuum manifested in the form of supernovas in the real world, so it's conceivable that if you could trigger a supernova (with trilithium, Tox Uthat, protomatter, etc.) while a Q was in proximity that might wound them.
Other godlike beings like Wesley and the Traveller or Kevin Uxbridge or Nagillum or the many godlike beings in TOS might have a chance.
And of course turning the Q against each other is always a possibility, as they seem eager to punish outliers of their kind for breaking established norms, and will either depower them and leave them to fend for themselves against the beings they have tornented, or they will exile them into a comet, or they'll send a tornado to destroy one specific home with two depowered Q (but miss noticing their daughter, so not omnipotent).
Voyager was shown how to enter the Continuum by the woman Q, so the means to establish communications with them instead of just waiting for one to appear exists. Just no one wants to deal with the Q, so they don't mess with it. Plus, it was very difficult to do, even with her help.