First of all why does that bother you? Is this person hired by your company and working in another company? If yes then i'd advice you to give him a warning first. If this person is hired by another company and now working in your company then i'd advice you to ask him to get a release from his old sponsor if you need him to continue to work for you.

Either case lets not make haste to complain and put our foot in someones daily bread, we are all here to work :)