(Rather like how lots of non-US banks block US nationals from having an account even if they're resident in the bank's country, because of the paperwork overhead)
It's not the same thing as a bank in another country refusing to open an account to an average Joe because of too many hurdles involved with reporting your balance the the IRS.
However, the 2nd and 4th Circuit Courts of Appeals have ruled that "blocking individuals because of their political expression constitutes viewpoint discrimination". However, the troll is being blocked because of past ToS violations. The system could block their ability to comment on any thread. Then they'll need to create a new throwaway account, which limits their ability to comment on government threads until a certain karma level is reached. It's not "discrimination" but rather "safeguarding". They can challenge it in court if they like.
If you have users with the required karma level trolling, they'll be downvoted anyway, so just mute their posts or hide them under a collapsed flagged thread.
This is quite different than the Trump case removing users' comments from Twitter.
> U.S. District Judge Naomi Reice Buchwald in Manhattan ruled on May 23 that comments on the president's account, and those of other government officials, were public forums and that blocking Twitter Inc TWTR.N users for their views violated their right to free speech under the First Amendment of the U.S. Constitution.
https://www.reuters.com/article/usa-trump-twitter-idINKBN1KV...
In the above case, the block was executed by the account holder who was a government official. Having the service provider flag a user as a troll and degrading their experience might be acceptable for comments on non-government accounts, but if a federal judge "flags" government accounts as public forums in a US court of law, then the service provider is now creating additional friction for a user participating in a public forum.
We have had political candidates and politicians like Alexandria Ocasio-Cortez doing AMAs on Reddit, so what I am describing is not a far-fetched hypothetical: https://old.reddit.com/r/SandersForPresident/comments/6ftvhu... .