Hi! Mapachelunar
We understand this issue. We have measures to ban (for different time periods, depending on the situation) players who try to rig their MMR
It's important to mention that we consider these cases on individual bases; we don't have an automated system that fully bans people because there are plenty of factors that affect this issue.
What we usually suggest is: if you see another player engaging in unfair practices: such as surrender or suicide to lower their MMR, for example, please, please, report this to troubleshooting with their usernames so that we can look deeper into their case and take appropriate measures.