Restricting RCON access?

I know form long time ago when my friend was server administrator he has restricted RCON access. I do not remeber right but i know he was able to login to RCON and his permissions was like file explorer files (read only) if i can explain good, or he used other rcon login details different form original credentials if i remember right. Ideas how to do this. I found something in RustAdmin called remote webclient in setting tab but i am not sure. for exmaple if want someone to check chat via RCON, players and stuff, but limit only to read and watch (No ban, spawn stuff use commands att all or only selected commands.)
Thanks in advance.