you can hedge, but the margin for the initial position is still in effect. The hedge costs 0 margin. There are 4 types of brokers regarding hedging:
Hedging not allowed. If you try to hedge, the original position is closed out. (most? all? ECNs work like this and some brokers too)
Hedging allowed. The hedge costs same margin as a normal position.
Hedging allowed. The hedge costs no margin, but original position margin still in effect.
Hedging allowed. The hedge relieves the margin requirement for the initial position. Therefore if all your positions are hedged, your used margin is 0.
