Yes, blocking LOS is one way of restricting Forge use.
It works particularly well with Wall of Thorns + Force Push as it gives Wall of Thorns another use.
E.g. enemy mage is activated in Forge Zone, cast Thorns on a border then Force Push him through
Usually a Forge is placed in Near Centre or Far Centre so that it can summon in 10 zones.
Placing it in start corner, it only works in 6 zones so you should Teleport him to your horde
Then restrict his movement back to his Forge zones (Force Hold, Tanglevine, Vinewhip etc)
Common Forge removal methods include:
Piercing Strike + ~6 dice attack
Bear Strength Brogan Bloodstone
Acid Ball + Surging Wave (not great)
Good luck with the next match!