checkval and != op

Hi to all, I've configured freeradius with mysql anche checkval and all works as expected when I try to authenticate users with Calling-Station-ID and := operators in radgroupcheck but I cannot use it with != operator. To sum up I want some accounts to login only on some machines and I don't want other accounts to login on the above machines. Can I use checkval also with != operator? ...if not, is there another way? thank you, Christian