Non members join your public games and miss turns. In fact, a noticeable percentage don't even make a move ever, forcing everyone else to wait a number of days whilst the timewasters are kicked out.
My suggestion? Give them a Negative medal for each game they do this in, which cuts down their turn time by 2 hours on each occasion. So if they join 4 games then deadbeat, they lose 4 x 2 hours so if they bother to come back and start up another 4 games, they'll only get 16 hours to make a turn instead of the regular 24. If they keep doing it, in time it could lock the acocunt (or reduce it to a far less inconvenient minimum). Once they complete 4 games then the Negative medals can start to be removed.