Nope - this is a limitation in InSim, not LFS_External. My only suggestion would be to select the player and then present the kick/ban/close buttons for that user only.
Yes - although I don't know the LFS_External API I can tell you that you need to send an IS_BFN, with sub type (SubT) of BFN_CLEAR, which will clear all buttons made by that insim instance.
I'll try to find a method for this the next days...
As I say, I don't use LFS_External so I honestly wouldn't know.
He helped me with it too...