See how this works for you:
:local name
/ip hotspot
:foreach i in=[/interface wireless access-list find authentication=yes] do={
:set name [/interface wireless access-list get $i interface];
:if ($name = "(unknown)") do={
/ip hotspot remove $i;
:log info "Wireless Access List Interface # $i Removed";
};
}
Edit: This will only work for interfaces that use Authentication, which I chose as a 'common denominator'. You can change this to whatever common denominator you wish, but I'm sure that one will do fine.
Enjoy!
-Krige