if you shoplift something from a grocery store and got banned, is there a way to be unbanned and come back?

If you re-enter the store and they don't say or do anything about it, it's the same as welcoming you back and renders the ban useless...however if you try that and they call the police, you will be arrested and face criminal charges. Is that a risk you want to take? Best to stay away from the premises completely (bans usually include the property the store sits on too).