Add "block user" feature
This would more effectively allow people to avoid arguments but has some potential downsides worth considering as well, e.g. users getting trapped in a filter bubble. However, other than for moderators, that is probably not a major drawback compared to the benefits, IMO.
How deep the block goes is also something that probably needs to be investigated and discussed. E.g. Does blocking a user just prevent PMs? Does it prevent their replies from notifying the user? Does it hide their comments/topics, and if so does it hide all the replies to those hidden comments as well? Etc.