[CalvinC]

Use a boolean array to check, first declare a global one in the top of your script:

pawn Код:

new bool:MinigunDM[MAX_PLAYERS];

A boolean can have 2 values, true/false (set to false by default), so in your command, set it to true.

pawn Код:

CMD:minigundm(playerid, params[])
{
    if(MinigunDM[playerid]) return SendClientMessage(playerid, -1, "Type /leave to leave minigun DM");
    // If MinigunDM is set to true, stop the rest of the cmd using "return", and send the player a message
    MinigunDM[playerid] = true;
    // If the command hasn't been stopped, it will set MinigunDM to true
    // Other codes
}

And then set it to false in your /leave command if it's set to true.