It really depends on the speed setting under interface dot11radio 0. Each 802.11b and 802.11g speed can be set as disable, enable, or basic(required). If you disable all 802.11g speeds (i.e. 6M, 9M, 18M, 24M, 36M, 48M, and 54M), all 802.11g clients should associate as 802.11b. Of course, you need to set at least one of the 802.11b speeds (i.e. 1M, 2M, 5.5M, and 11M) as basic(required)
If you disable all 802.11b speeds or make one of the 802.11g speed as basic(required), all 802.11b client cannot associate to the WAP.