You want to disable user from Local user list on Proxy

You will need CLI access for this. You can follow below steps to disable user.

• ProxySG#conf t
• ProxySG#(config)security local-user-list edit ?
   <local user list>

• ProxySG#(config)security local-user-list edit testlist

  testlist is name of  local user list

• ProxySG#(config local-user-list testlist)user edit ?
   <user name>

• ProxySG(config local-user-list testlist)user edit USER1

  USER1 is test username which you want to disable.

• ProxySG#(config local-user-list testlist USER1)disable
    ok