[Mauzen]

With checks I mean for example this: On your gamemode start, you start a repeating timer with e.g. 2 seconds delay. This timer loops through all players and checks if a player is in that area (so almost like your warning, but this would be the first check). The difference is, instead of kicking the player directly if he is in the area, you call another function (best via SetTimerEx, you can add the playerid to the parameters then) and can send a warning that he has to leave.
The other function would have 15 seconds delay in your case. It then checks again if the given player is still in the area. If he is, you can kick him now, because he is still in the area after 15 seconds.