[Exhibit]

only once?
You can make a variable like this

PHP код:

new UsedCommand[MAX_PLAYERS]; //on top of the script
OnPlayerConnect(playerid)
{
    UsedCommand[playerid] = 0;
    return 1;
}
CMD:yourcommand(playerid, params[])
{
    if(UsedCommand[playerid]==1) return SendClientMessage(playerid, -1, "You can only use this command once!");
     // rest of the command