Void Mage doesn't remove a signet, it removes a modifier or ongoing effect. It is generally used to flip the outcome of an encounter.
It is not really overpowered because to use the effect you need to lose the encounter for it to activate and it requires a proper setup where it can flip an encounter. There is not always a good setup and it can take some skill to set up Void Mage to be useable and good opponents will often tie with their own Void Mage to block.
Say the game state is tied 3-3 for signets and you sneak in your Void Mage to flip one of the previous encounters. Your opponent wins the Void Mage encounter so now you have a 4-3 signet advantage. You gained 1 signet and denied the opponent the chance for an ability.
The best outcome you can get is removing a judge (8) effect on a heavily tied board. That is the only way I am aware of that will change the signet outcome by more than 1.
