Why not just ask nicely (which has been done before), and if he says no, ask him to place a big, flashy, gawdy, super-duper shiny box near the top of his pages explaining that he is not the _official_ ombudsman with a link to the real users' pages. And if he refuses that, tell him sorry, block the account based on an inappropriate username, and be on with it. Too harsh? --LV
Well, there is no reason to block his account and deprive him of his past edits, whether he complies or not. We will simply have to force rename him (User:The User Previously known as Ombudsman).