Use TPXOPER command:
MOD VT=vtermid,{AVAIL|UNAVAIL} allows you to specify whether the designated virtual terminal is available/unavailable for selection.
Release: 5.4
Component: TPX for Z/OS
From TPXOPER issue the following command:
MOD VT=VTNAME,UNAVAIL
The above command will mark the virtual terminal as unavailable until it is made available again by using TPXOPER or TPX is recycled.