//At top
new BankRobbed;

//Add these check in command
if(BankRobbed == 1) return SendClientMessage(playerid, color,"Bank already robbed return later xD"); 

//In command when bank is robbed (foe example before SendClientMessage(playerid, color,"You robbed bank"); 
BankRobbed = 1;
SetTimer("ResetBankRobbery", 28800000, 0);//28800000 is a 8h timer

forward ResetBankRobbery();
public ResetBankRobbery()
{
    BankRobbed = 0;
    return 1;
}