Another method could be used too, but means more work:
Set up a second allied AI "country", that is actually the same as your first country. Ok, sounds quite confusing. Here's what I mean in an example:
1. Apart from the human and the AI controlled sides, use another AI side that has the same country as the player. See pic below, there we have a human-controlled Soviet side (green), plus a Soviet Ai in orange, both allied (same team number)

- AI_side1.jpg (372.89 KiB) Viewed 1114 times
2. To make this work, deactivcate " Teams share victory points"

- AI_side2.jpg (231.56 KiB) Viewed 1113 times
3. Now you simply set all Soviet territory to green. Then give a few flagged hexes to the orange side, and the player cannot deploy anymore there. The flags are sill visible, and since both green and orange Soviet sides are allied there is no visible front line between green areas, and orange-owned flag-hexes:

- No_deploy.jpg (459.19 KiB) Viewed 1113 times