[elci]

this in the top of your script:

new in_dm[MAX_PLAYERS];
this in OnPlayerCommandText

if(strcmp(cmdtext,"/dm",true));
{
dm[playerid]=1;
}

put this where you wanna block command:

if(in_dm[playerid]==1)return SendClientMessage(playerid,0xFF0000FF,"You Cant Use Commands In Dm Area!!");