You're unable to force the system to allow you to stop people editing your posts, this is a feature of the StackExchange model, that anyone in the community is free to improve upon posts (once they reach a high enough rep to do so). Generally, its a great system which ensures the quality of questions and answers here.
Usually, edits are for minor things such as spelling, or adding MathJax formatting, and should be preserved, however, I grant that there are some users here who edit excessively (if you search meta there have been several recent discussions on this).
If you are against someones edit, you can re-edit your post, or roll-back the post to its original state. Generally once you do this, the person who edited it should get the idea and not try to change it again (avoiding so-called 'edit wars').
Do be aware though, that if you're choosing to roll back perfectly good edits which leave your question poorly worded/formatted, then people are also quite within their right to vote to close.
Also note that until you have a certain amount of reputation, your edits will be placed into a queue for moderation (this is to prevent new users from trashing the site). Once you have sufficient rep, your edits will be applied instantly.