A few of the main results when you ignore a user:
- their post replies are hidden from direct view
- you will not be notified of their replies
- they cannot send you private messages. This includes creating and replying to you.
There are other actions that happen, but I dont recall what they are at this moment.
If an unignored user quotes someone you are ignoring, the text will be displayed normally because it is a complete post created by an unignored user.
If there is a questionable user, post, or thread, it should be reported to the forum moderator so it can be dealt with.