I have been on PredictIt for a couple months for the US elections. The site needs major policy changes to survive.
It is infested with trolls who
constantly post so that is impossible to follow topics of interest because one has to scroll through junk,
take pseudonyms like Trump, Clinton etc.
1. Require people to post under the name that they deposit funds.
2. Allow buying and selling bets for any member but install a switch to only allow posts when holdings exceed $10.
3. If possible limit postings to five per market per better per day.
As it is, the screaming outweighs the value of the boards. I can't hear people think which is necessary to improve the betting. I plan to leave after the elections.
Your problem is with the commenting system. It uses the DISQUS commenting system that many sites use. As such, limiting it to PredictIt logins only is probably not possible, much less limiting posting or restricting it to people with certain holding within PredictIt.
But making use of the user ratings helps. You can sort by most up-voted, to help ignore the trolls. And help set those ratings by up-voting and comments that you find thoughtful or helpful, and down-voting the trolls.