Hi! Forgot to respond to this.
Here’s a video illustrating this: https://imgur.com/yOCJxa2
Just hide all the tags that you don’t want appearing in the filter. You can even select the checkboxes at the left of the tag names to hide multiple at the same time.
If I’m not mistaken, “Filter by Maintag” just means that for posts with a lot of tags, Publii will just take into account whatever the set “main tag” is when filtering those posts.
For example, if Post A has Tag 1 and Tag 2, but only Tag 1 is set as the main tag, Post A will only appear when the user clicks on the Tag 1 filter, but not for Tag 2. However, if you turn off “Filter by Maintag”, Post A will appear when a user filters for Tag 1 and Tag 2.