physical trauma is actually one of the best methods of euthanasia available to private aquarists, if you do not have suitable anesthetics on hand. but many people are squeamish about such methods.
freezing is a slow, painful process of death for your animal.
and apart from option #2 here, of which i am not certain, these are in fact very good choices.
overall, using a tried and tested chemical will be the best way to euthanize your fish.
just to add, NEVER release fish from your tank into the ocean. even if caught from the same waters. even if there are populations of the same species in those waters. in 99% of cases, once a fish leaves the ocean and is put into a private tank, it is not suitable for reintroduction into the wild. (remaining 1% reserved for scientific institutions who work at the highest level of scrupulousness)