I do not know how to ignore people on this forum, all I can do is not to read the person's post if he is spamming. Another reason that I think people can put someone on ignore list is if they are arguing, I prefer not to put the person on ignore list. Admittedly, some people are really worth to be ignored.
If you look closely under the avatar of a user you can see there is a Ignore link. Of course you just need to click that

As for me I do not have anyone being ignored because I want skip bad posts fast. Only if it's a full spam of crazy words I would ignore someone