Question: I edited someone's post (<https://stackoverflow.com/review/suggested-edits/8559008>) to correct some grammar and spelling mistakes. I also removed the kinda useless phrase "Please help me!!!!".

It got approved by some user, but then the OP rejected it with "This edit defaces the post in order to promote a product or service, or is deliberately destructive."

This was the first review that user (who has currently a total reputation of 1) did. I don't think he even knows what the rejection reason means.

Is there a way to dispute a rejection like that?

Answer: 
If an author is rejecting appropriate edits, or rolling back edits that are very clear improvements to the post and edits that are appropriate as per the site's editing guidelines, then flag the post for moderator attention. 

The user is vandalizing their own posts, which is not appropriate behavior. 

It's not a situation that comes up all that often, so there aren't really tools for non-moderators to deal with the situation. Mods, on the other hand, *do* have tools to deal with users vandalizing their own posts, so if you explain the situation in a flag they can look at the situation and take whatever action(s) they feel are appropriate.
As the author I feel I should have a final say on what I sign my name under. That's why the author's vote is binding. I think that's a good thing. People can suggest changes, but in the end it's your post. I feel that in order to be comfortable with other people changing my posts, and those changes possibly taking effect without me knowing about it, it's important that I can override/change them. 

Obviously that might sometimes mean people seem to hold on to something you really don't understand, or don't appreciate, but in the end it's all about striking the right balance, which sometimes means letting go. Better to have to let go of someone else's stuff than of your own.