Use either the Does Not Stack trait (can make the marker static after placement and keeps it from stacking with other things), or put the marker in a separate Game Piece Layer that draws below units and by definition will not stack with them.
The Does Not Stack trait has some drawbacks. As soon as you think it’s a good idea to make a piece totally immobile, someone will want the ability to do a takeback or change their mind about where to put the thing. So then you add the modifier to only be able to select/move the piece while holding down a key (Alt, Ctrl, or Shift)–but this tends to be completely opaque to users unless you document it very prominently, so people will end up complaining they can’t select/move the thing, not knowing that holding down a key is the way to do it.