Hardy, to answer your question, ULib uses standard Source/SteamId bans.
Once a player is permanently banned/kicked, ULib never sees that player again.
As MrPresident explains, simplest terms; Lua is one of the last things to 'see' a player when they join a server.
AMX sees them (I believe) right as the server does because it is running as a dll/binary along side the server code.
ULib doesn't use binary code running on servers like AMX does, it uses Lua, which is initialized _after_ a player has connected.
I don't believe it would be possible to even write a lua script monitoring system to look for the previously banned players... I don't think the banned player attempting to join sets off any hooks.