The difference between ignore and block is with ignore, you've chosen to not see content by X user. This typically works on most sites and it why it is the default standard on all major mainstream developments such as
Block is a completely different function in that it does more than just ignore, it actually prevents the other person from replying. For many sites this would be counter productive because you want to encourage people to post and add content.
However there are a few occasions where this maybe needed on some specific sites. So I would suggest this as an option.
It would need to be added to the core, because it would be useless if it could not be applied to ALL Woltlab products.