No it is not, however the workaround would be to drastically increase their price so they can't be bought

Not directly but I found a workaround recently. You can use the Parameter "BuildingFrom" to set eg. a Thanatos to be built from the SoldierCenter. When loading in, the Thanatos will no longer show up in the Engineering Center but also not in the SoldierCenter. Based on numerous tests it doesn't seem possible to reverse it (as in, make the Thanatos appear again). I haven't tried if its possible to do this with buildings but it should be the same principle.

To add to this, you can use the Parameter "PanelPosition" to shuffle the unit card position in the production tab. For reference, Rangers, Soldiers and Snipers have PanelPosition values of 1; 2; 3 respectively, with 1 being most left and 3 most right. Shuffling these 3 values around will move the unit cards in respect to those values.