From my turns on festival stewarding, I often get stationed at the back side door of the main concert hall. We put signs on the door asking people only to enter or leave the hall during applause/breaks between songs or sets, and the MC usually also makes a polite request to this effect at the start of the concert. We have come to rhe conclusion that in spite of this, you really cannot stop anyone determined to get up and leave, but we do try to stop anyone re-entering until a suitable break. The door itself can open outwards or inwards, but if the latter, it comes up against me and some big heavy curtains! So, in very big letters on the outside of the door is "PULL!" but guess some people just can't read!
In another place I nearly had my arm broken by a very drunk woman after I had placed it across the door: she was intent on barging in in the middle of a song, and not going to take no for an answer from several of us patiently waiting to go back in at the end of a song. She was noisy and very rudely spoken too!