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pawn Code:

new ChosenSkin[playerid]

Under OnPlayerRequestClass

if(ChosenSkin[playerid] == 1)
return SpawnPlayer(playerid);

Under OnPlayerSpawn

ChosenSkin[playerid] = 1;

Under OnPlayerConnect

ChosenSkin[playerid] = 0;

That means when they first connect, they can chose skin, but after they've spawned, cannot. Not tested.