People use their block lists for different reasons. Here's what the effects of blocking some are:
1. Their reviews will look
to you as though you had rated them NH: they will filter to the bottom of categories and, if you use your content filters, you won't see their content at all. That means that you don't even need to see comments or ratings by them, if you set your content filters not to show members you've blocked.
2. You will be contributing, in a very small way, to the overall community's view of that member. If a lot of members block member DeservingToBeBlocked (I assume that's not anyone's acutal member name!), then the member will be considered "distrusted by the community". This means that their content (reviews, ratings, comments) will be invisible to non-members and invisible to members who choose to set their content filters to filter out members distrusted by the community.
You can decide for yourself what constitutes a good reason to block someone. The sorts of reasons that some people choose include:
* Plagiarism
* Really offensive content. You'll know it when you see it.

* Abusive behaviorm towards other members (really nasty comments)
* Consistently inappropriate reviews (blatantly off-topic, utter drivel, content that reaches some threshold of "bad")
It may take only one incident to make you decide to block someone. It may take a sustained pattern of behavior. You'll really have to figure out on your own what your standards are.
But you should know:
(1) no one else knows when you've blocked someone (including the blockee)
(2) you can block someone and still view their content if you choose to -- just set your content filters to see it
(3) your decision to block someone won't affect the community until a
lot of other people block them as well