There are several ways you can completely deny incoming aircraft on the tower and radar frequency via the miscellaneous command sub-menu.
“Deny Entry” says “N623KB, Due to heavy traffic, airport is not accepting incoming traffic at this time”. Translation; come back later or divert, but we probably will definitely not be able to accept you any time soon.
“Unable, Divert” says “N623KB, Airport is unable to accept arriving traffic. Please divert to a suitable airport”. Translation; we’re not experiencing heavy traffic per say but we can’t accept you at this time, go somewhere else. You will never be able to land here.
“No Light Aircraft” says “N623KB, No light aircraft accepted at this time. Please divert to a more appropriate airport”. Translation; you can technically land here but your aircraft is so incredibly slow that you’re becoming a major problem, so land somewhere else.
All of these commands are subtly different but can have their own individual uses.