How can I remove or modify the properties of certain decals (Signs/Explosive marks) in TF2?

I found out something that may benefit you while researching for this answer. You can reduce how many decals in total that are active at one time using the mp_decals configuration setting.

By default, this is set to 200. You can reduce it to be as low as 1 or as high as 4096:

The maximum number of decals that can exist at once. Decals are used for visual effects like bullet impacts. Higher numbers will result in more persistent decals, at the expense of performance.
_{TF2 Wiki - Multiplayer Options}

So, if you want to reduce the amount of black explosive marks on the ground, it's likely that setting

mp_decals 1

will do this for you.

There are plenty of other tweakable options/configs and many of them have to do with decals, however not all are well documented, and there are too many to test out. I would suggest playing around with them to achieve the desired result. However, do note that some are considered 'cheats' and thus will not work on Valve servers or servers running sv_pure 1. Check out the List of TF2 console commands and variables from Valve.

Thanks for the answer, Robotnik. I played around with some more of the decal commands and I figured out r_renderoverlayfragment is the command to disable or enable all the signs around the map.