There are a couple of possible solutions for you. First, other people cannot edit a document while that document is actively being edited by another user.
Second, if you are looking for a way to lock other users out of editing a document at any point, check out the "Collaboration Options" section in the "Users who may edit" sub-section. There you can limit who can edit the document to yourself, a list of specific people, or open it up to anyone who has the edit permission on documents (the default).
I hope that helps. Please let me know if you have any other questions.