Ideally the people of Ukraine. It really should be for foreign powers to say. I mean you realize many government have provisions to handle ousting members and reforming, right? Particularly many parliamentary systems, but others as well. Even the US has something via the 25th amendment. The vice president and either a majority of the cabinet or congress can declare the president unable to discharge his duties and oust him (go read it if you want more specifics). It isn't a situation of "Well they were elected, now they are there and there's nothing you can do until the next election!" There are processes for removal/recall in pretty much all political systems.
I don't know anything about Ukrainian law, but really, it is for them to decide internally. Their courts need to rule on their law. It is really up to the US or Russia to come and say "No we don't think you should have done it that way, you do it our way instead."