Used to work with a lad who used to tag along when we went to the pub and he would either slope off before it got to his round or if he actually bought a round he would stay to get all the drinks from the second round and then go before it was his turn to buy his second round.
He also used to tag along at lunch time and then say "I've left my wallet in the office can someone lend me some money to buy my lunch".
Everyone just told him to do on after we sussed him out after the first few weeks.
One time he tried to leave after being in a round with 8 people and he got told he was going nowhere until he bought his round. We went from a cheap pub to an expensive one up the road and all ordered really expensive beer to teach him a lesson. He was nearly in tears when he got the bill.