It's not too hard. Click New Private Message If you haven't done so, you'll need to click "Find new buddies" and select the screename of the person you want to p.m.
After you've added contacts. Select the contact(s) you want to p.m. by clicking the "add" button. Click "invite members" and then confirm posting privileges and members. You should then be at the message box. Compose a message just like you do in this forum.
Another way ... if the person has a post on a thread ...
left click their name and it will give you a drop down menu. One of the items is "invite _ _ _ to a private topic". It will immediately give you a window to type your message.
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .