Two-Day Cool Down Before Being Able to Join Another Clan
To prevent clan-hopping, it's best if players are subjected to a two-day cool down before being able to join another clan. This way they will think better about their choices of joining a clan instead of bouncing from clan to clan every couple of hours.
Meet the Needs of ALL Players
How about a 8-12 hour cooldown for joining clans?
That gives a huge incentive for players that join in the morning to at least stick it out for 8 hours of the day before they quit in the evening (if they still want to quit). That would allow players to join up to two clans per day (unless staying up 24 hours, which would be 3 clans/day), and also severely decrease the chances of getting a clan hopper who joins and leaves within 5 seconds (not exaggerating).