Unfortunately, there is not a way to prevent others from changing your posts. The whole point of SE sites is to allow the community to monitor and modify content where appropriate. Sadly, this sometimes means that unnecessary or incorrect edits may end up getting made. I believe you should see a Rollback link below your post. If so, you can click that and it will remove the edits that were made and restore your original content.
For others who may happen to read this, please consider using the Comments area for radical changes to a post. If you are making simple grammar or spelling changes, that's not a problem, but even then you should leave a comment on the editing screen to indicate what you changed and why. If you see something you deem to be considerably inaccurate, then comment on it and let the community help decide how to handle it.