i tried it and could remove the signs, but with one different problem: since they don't stack, i quickly fill my inventory with them after removing a big smelter area or a bus. can you please make them stackable? and since they are intended for purely cosmetic reasons, there should also be no need to limit stacksize to 1, 5, or 10, but they could just stack "unlimited" (eg 100 or 400)
to not mix them up with "real" items in the inventory, the icons in the craft window and in the inventory could/should get some variation: no need to create new icons, but please give some tint to the icons (that's just 2 or 3 lines of code) to easily see that it's a "iron-area marker" and not an iron plate.
and finally another suggestion: these signs nicely fit in a square of the grid, eg for my gridbased train system that consists of 3x3 gridblocks for each "cell" i can mark either all 9 or just the center. but when i plan other things like a smeltery or science production, i often build them only half a gridcell wide and two or more gridcells long.
for other people would it be too much work if you just halve the size of the signs so that a grid cell needs 4 signs to cover it completely? that also would allow easy marking of other shapes, eg a big starter smeltery with 1 copper row (1x4 signs), 2 iron rows (2x4 signs) and one shorter but wider steel row (2x2 signs). the most extreme case i encountered with some test setup: a row of 128 assemblers to make green circuits which was only half a gridcell high but 16 gridcells long :-)
on the other hand, we still can use the old vanilla method of making one blueprint and putting that one down, and thus at the same time also having exact dimensions and not only a general layout
