sadly this is due to how mIRC is coded
No it isn't, if it were, it would freeze up all mIRCs. It does not freeze up mine and I can't remember it ever doing so. It usually lags you on big networks if you have a slow internet connection, which also hasn't got anything to do with how mIRC is coded.
There is also no command in the RFC (I dont think) to tell the server to halt sending data to furfill a previous /list request (I may be wrong but im almost positive theres not).
If you mean stop listing once you've typed /list, there is
/quote list stop and/or
/quote list halt. You can also right click on the list and click on 'Stop Listing'. This doesn't usually happen instantaneously.
Regards,