Hi Toni !
The most certain way to make it work only for 11g clients is to disable all 11b rates on the AP (e.g. 1,2,5.5 and 11 Mbps), or to set at least one 11g speed at required (6 Mbps) and 11b speeds to enable (no basic/required), yet the latter might not work with all software releases.
Best Regards,
Mark