Can I use this sign for a toy block?
No, this specific sign means to prevent, hinder, or obstruct an action. If you want to talk about a toy block, a building block, or a square shape, you would use both hands in 'C' handshapes to physically outline the shape of a cube in the air.
Does it matter which hand acts as the barrier?
Yes, it is standard practice to use your non-dominant hand as the stationary barrier. Your dominant hand, which is typically your more active hand, should be the one that moves forward and is stopped by the barrier. Reversing this can feel awkward and look unnatural.
How hard should I strike my hand when signing?
You do not need to hit your hand forcefully or cause yourself pain. A firm, crisp tap is all that is needed to effectively convey the idea of an action being abruptly stopped. Focus on the sudden stop rather than the force of the impact.