Reset button "Clan Wars" to OFF each time a league starts
There are many players with the "CLAN WARS" button set to ON by default but they actually don't attack in clan wars. A way to control AFK players (or partially AFK) could be to reset the button to OFF each time a league starts. If the player really wants to be a member in a war he has to put it to ON again! (Another possibility is that the leader could modify the current status of each player).
The current status is misleading, there are player that set it once to ON and it is currently ON but they don't actually want to be members in a clan wars.