In 4.2 code, "AP Group Vlans" only overwrites the vlan assignment of the WLAN. It does not change what ssids are on an AP. So if a WLAN isn't listed in the AP Group, it just takes the default.
What you are trying to do is a wlan override which is done per radio in 4.2.
I think you'd going to "Wireless > Radios > 802.11a > edit one of the APs > some kind of option for wlan override > uncheck the box of the ssids you don't want"....
I think it was 5.2 where WLAN override merged with AP Groups (which makes it 100 times easier to configure and manage).
Perhaps WCS has a template or something you could use so you don't have to manually edit each AP radio?