Well, you can't.
But you can achieve the same result by hiding the Groups or Tags (categories) you don't want to see.
To hide Groups: Right click on any group and select "Visible Groups....". Then click on the "eye" icon to toggle it's visibility.
To hide Tags: Right click on the group that contains the tag(s) you want to hide and select "Add or Edit Tags....". Then click on the "eye" icon to toggle it's visibility. The "Other" tag has a special purpose. It groups all the hidden tags into one tag. You can control it's visibility the same way as a regular tag.
