There is a command hidden in plain sight in the miscellaneous command list called “No Light Aircraft”. The command says “N623KB, No light aircraft accepted at this time. Please divert to a more appropriate airport”.
According to the manual in regards to adding No light aircraft in ATIS, “This command must be used at ALL times for large airports that do not accept light aircraft IRL (e.g. EGLL, OMDB, VHHH etc). In addition, it is important to note that this command is not traffic dependent – a consistent message must be sent that there are other, more appropriate airports for light* aircraft”. But there’s a loophole here.
That just covers what can and cannot be allowed in ATIS. At any time you can deny light aircraft individually using that miscellaneous command found on Tower’s frequency. You must always try to fit those slower performance aircraft in but if you’re unable to, which is rare, then you have the ability to deny them temporarily.