You have to write an on load client script on sc_req_item table.

function onLoad() {

    //Type appropriate comment here, and begin script below

    if(g_form.getValue('cat_item')=='sys_id_of_catalog_item' && g_form.getValue('assignment_group') !='sys_id_of_assignment_group'){

  g_form.setVariablesReadOnly(true);

}

}

Write down the appropriate sys_id in place of bold text