ThinLinc local drives redirection relies on being able to ask the kernel to mount a NFS share. This is a privileged operation that only root is permitted to perform, and as such this feature requires a setuid helper binary. This helper is called /opt/thinlinc/libexec/tl-mount-personal and the setuid permission can be removed by running the follow:
$ sudo chmod u-s /opt/thinlinc/libexec/tl-mount-personal
Note that this only disables the ability to use the kernel NFS client. If a user can start some other NFS client then they can still access the local drive redirection. The setuid permission is also restored each time ThinLinc is upgraded.