Markers cannot be turned off from the tactical map, and even if it would be possible, only enemy markers should be turned off. Friendly markers should stay. Also - in addition to time, direction, spot and enemy type - showing spotted enemy strenght would be very important.
At least for me this would be a good step towards increasing simulation value of the game. There wasn't a GPS map back then.
Some other additions would also be nice regarding to this markers removal. Here's a few suggestions / ideas:
1) Player (as a commander) could request an update of enemy information / movements => all the enemy contacts (time, direction, spot, enemy type, strenght) seen by friendly forces could be updated on the tactical map with a slight delay.
2) On the tactical map, friendly units engaged in combat could be visualized.