[Denying]

Just create a faction members file and add a player's name into it uppon faction invitation.. and remove the name uppon a kick...

PS: Make sure you remove all the faction members from every faction unless you add them manually.
Because if you add their name there uppon invitation the members that are were in the faction before that update will not be in the list.

EDIT: Example for a faction-members file...

FactionName_Online_Members.ini

But you can call it however you want as long as you know which file it is.