I've been playing with my friend in Knight Club for quite a while now and I feel that the shield is very spammable. In fact, many times we've just stood still with shield and still be able to block all attacks when aiming the right direction.
My recommendation is that the shield has durability to reduce shield spam. My thought is that the shield has 3 charges and loses a charge every time it gets hit. Every 5 seconds a charge will be restored if it isn't already at 3 charges. Once the shield charges goes down to zero, the player shouldn't be able to use the shield until it's all the way charged up(Kind of like a gun overheating in which you have to wait until it goes all the way back to 0 heat).
Hopefully you could look into my recommendation, Gutter Arcade.