[Andom]

Top of your script:

pawn Code:

new BankIsRobbed;

When somebody is robbing:

pawn Code:

BankIsRobbed = 1;

When somebody is done with robbing:

pawn Code:

BankIsRobbed = 0;

When somebody wants to rob (/rob or something?)

pawn Code:

if(BankIsRobbed == 0)
{
  // Here your rob code
}
else
{
  SendClientMessage(playerid, COLOR_RED, "The bank is already being robbed by some guy.");
}