If you add the Group column it will hide the headers. It's under the General category.
You can also turn on Preferences / Folders / Folder Display / Add 'Group' column automatically when file display is grouped if you want it to always be used when grouping.
I should have stated this earlier, but I like the group feature because it creates a one line space between each "block" of files of a certain type (as shown in the second screenshot in my original post (where only the negative sign appears, without the text). Adding the Group column or checking the Grouping option in Preferences makes the group headers go away, but it also removes the one line gap that separates files of each type. Basically, it makes the Lister display files the same way as sorting by file Type does.
It's not a huge deal that the negative signs are shown in the Lister, but it would make the list look a little cleaner, IMO.
One way to make the text disappear is to make the text the same color as the background (in your case, white). Preferences / Display / Colors and Fonts / File group header. Though it still leaves the line in.