It has been a few years since I configured flexlm, but my memory is that you can specifically reject some users in group files.
My thought, though, is that what you want to do is remove them as authorized Named Users after having originally authorized them. There is a mechanism for that; the details might depend upon which operating system you are using.
Note for the reference of others: the MATLAB license terms allow you to change the Named User designation for any given license, but only a limited number of times a year for any given license. (a policy restriction rather than a technical restriction.)
