If you decide you no longer wish to have contact with another member of the site, the Ignore feature will allow you to do just that. They will no longer be able to private message you, their posts will be hidden from you (not completely invisible, just hidden in boxes so you don't have to read it unless you press the Show option on said box), and you will not get any alerts from them tagging you.
To use the Ignore feature, simply go to that person's profile, and look at the upper right of the page (above their profile posts and such but below the regular site menu bar, see image below). There are a few options there, one of which is Ignore. Click it and the word will change to Unignore, which signifies that you have placed that user on your Ignore list.
However, if you believe a member of the site has done something against the rules, such as if they are harassing you, then you can choose to report them to the Security Department of the Staff. Ignoring and reporting harassers is an ideal combination to get them out of your face and to make sure we know about it so we can make sure they leave you alone and don't harass anyone else. Reporting a member is just as easy as ignoring them: in the same place as the Ignore option, there is a Report option. It will give you a little pop up window where you can fill in the Report Reason, and that will be sent along to the Security team to be looked at and handled.
Alternatively, if you cannot or prefer not to go to the person's profile, you can use the Report a Member form. It will ask you to enter the member's user name and then fill in an explanation of why you are reporting them, which will then be sent to Security. This link can be easily found on any page of Iwaku by scrolling to the bottom and clicking the option from the list titled Useful Links.