Spoilers for The Day of the Doctor.
As far as I recall, the Zygons were suspending their troops into stasis paintings so that they could emerge from them in a future to conquer a less primitive world instead. Elizabeth, knowing this, placed the paintings under heavy security and asked for the Doctor's assistance in case something odd happened with them in the future.
Destroy the paintings. Reckon they have to break out of the paintings themselves (as they did), so I guess that destroying the paintings would kind of lock them up forever. Actually, this also makes me wonder, what happens when a stasis painting is destroyed? Could be another question.
Send the paintings to some world far away from Earth. Let the Zygons break out of the paintings - they'll just end up lost in some random place, with Earth safe.