Cloaked Wormholes?

[Suicide Junkie]

Some other things you could try are:
1) Sector damage ability. Not only discourages exploration, but will require players to research hullsizes and/or armor before they can explore very far.
2) Make many Warppoint lookalikes.
Changing the image of the wormhole to make it look like a storm could work until the player tries to warp through
3) Dummy warppoints If there are 25 storms in a system, and only 3 of them are actually warppoints, it will take players longer to figure out which is which.