Rodger Hoyles, 54, of Church Road North, admitted assaulting Derek Furnace by beating when he appeared at Boston Magistrates Court.
It was said that Hoyles went into the Poundland store at 6.20pm on August 13 and was advised by another member of staff that the store was due to close at 6.30pm.
The magistrates heard that Hoyles became agitated and abusive and picked up a shopping basket and threw it at Mr Furnace, a member of staff in the store.
When he was asked to leave, he punched Mr Furnace on the cheek before leaving and then, after the doors had been locked, stayed outside and when Mr Furnace opened the door, punched him again, causing a nose bleed.
It was said Mr Furnace went to the emergency department for treatment for a cut and bruising.
The court also heard Hoyles handed himself in to police the following day, saying he had been drunk and had been stupid and admitted it had been unprovoked, but the prosecutor said it had been a 'prolonged and persistent assault'.
In mitigation, Michael Alexander said Hoyles had had 'too much to drink' on that day and he had 'got annoyed when he was asked to leave'.
He said that when he realised the next day what he had done, he handed himself into the police and had now sought help with his drinking and mental health issues.
The magistrates said it was clear there was some remorse and ordered a stand alone compensation order to Mr Furnace of £300.