If you cannot get proof of his steam id then I would ask a staff member to see if they can find out when he next comes online. However until then, although this is solid proof there is no proof of steam id which means that a staff member cannot ban them as you may have given a false one.
Try and find some proof of it...

If you cannot get proof of his steam id then I would ask a staff member to see if they can find out when he next comes online. However until then, although this is solid proof there is no proof of steam id which means that a staff member cannot ban them as you may have given a false one. Try and find some proof of it...