It would be good but since many players do not read F1, We could like add a message that gets sent to criminals and to cops once the bank is started that "You may not use grenades out side the bank, or it will count as Death Match" or something like that.
Or Or even better, just make the bank a "Safe Zone" no access to gun and cops only can use there nightsticks and tazer to prevent abuse for criminals, therefor no one could do this I could try to script this im new though so..