You can't do anything about it, but wait and hope the unit will leave of its own accord.
In a dev stream Scott himself said that he hates it when allied units blocks roads and outposts, and are hanging around your cities blocking you. So they are aware of the problem, but when a fix will be forthcoming i have no idea.
I think I may have asked that question during that stream. From what I recall it didn't even sound like they were planning to fix that at the time. More like they were taking it under consideration. So not to say they won't, but it certainly didn't sound like something that made any formal fix list.
edit: LOL, just as I say this I now see a changelog (the same minute I posted!) for an opt-in patch that looks to maybe fix this!