Browser IPS signatures are by default Block and Log, you can create exceptions for them, but the only exception you can create is Allow and Do No Log, ie a traditional type of exception. This is due to limitations in the engine - if its going to detect, then it has to block.
Thaveshin, all the signatures are enabled in blocking mode by default, what are you trying to change?
Brian, the UI isnt particularly intuitive here.. we should work on that :)