I'm asking this because I came across the a user. If there is a function to report this kind of abuse, I haven't found it in the user's profile.
Please take this as an official complaint about the user name of user AdolfHitler.
By experience, there is no such way of reporting a user.
The way to do it is by flagging on of their post if possible. But I know, it's not always possible. The Meta question : Flag abusive users explains the situation.
You're supposed to flag what the user has done rather than the actual user. So if they've done a lot, flag it all. Not only will the system kick in (auto-bans, etc) but the moderators tend to notice.
If you want to get creative with your flagging, you can leave a custom message about the user in the Other flag type but don't use that if they're being offensive or spammy. The Spam and Offensive flags have special meaning in the system for preventing further incidents.
If you need something dealing with quicker or with an element of privacy, there are ways of talking to the moderators or SE directly:
@nametrigger to ping one of us and we can either talk in that room or start a private room.
Just remember that like you, we're just volunteers.
Flagging should be done based on dubious questions and answers. If these are out of order a lot it will lead to a ban of the user.
OK that name would not be MY choice as a name but the user, regardless of his personal opinion, should be considered a valid user until proven otherwise.
And now for the biggest problem: how far do you want this to go? Adofl Hitlar is a user too. Do you suggest to ban any combination of the letters that make up that name? Then what about Himmler? Or Stalin. Both where by far worse than that other idiot ;)
All what some people want to do is annoy, insult etc you. If you do not act they will not get their satisfaction.
So I guess: let them choose whatever name they want. Knowing their attitude they will get banned soon enough based on their posts (the 1st almost always gets followed by the 2nd).