If I already have belts and inserts on my main hotbar, I don't need them showing up here as well. Just an idea to reduce clutter!

Thanks

There is to ways to interpret this request, and I would like to make both an option:
1 - Hide anything on currently shown quickbar rows from showing on the Placeables GUI
2 - Hide anything on any quickbar row from showing on the Placeables GUI

I believe you are asking for the first option. Unfortunately option 1 is currently a lot more difficult to implement than option 2 due to performance issues because I cant not easily tell when a player changes the displayed quickbar rows.

I submitted a request for a addition to the modding API to make implementing option 1 more possible: https://forums.factorio.com/viewtopic.php?f=28&t=89510
Even if the request is approved, it wont be available until the next update to Factorio.

Option 2 is a lot more possible though, and I may eventually start working on getting that feature available.

also waiting for this