How do you deal with developers that overengineer?

From all the software acronyms we learn along the way, KISS for me has shown to yield the most workable and maintainable code. How do you deal with over-ambitious colleagues that drop PRs full of unnecessary structures and patterns? I mean it does the job, but in 4 months time when we have to extend it or fix a bug it's going to take us 3 days to reverse engineer everything. While if we fix the thing simpler and less academically fancy way we can just read what it does.