You can block another user from sending you scraps, messages, invitations, friend requests, and testimonials by adding that user to your ignore list. The ignore list is visible to you and other users will not know who is on your ignore list.
To ignore a user, visit their profile. Hover over the more link in the left sidebar and click ignore user.
You can remove members from your ignore list with the following steps:
1. Visit your home page and click lists on the left sidebar.
2. Click the ignore-list tab.
3. Click delete next to the name of the person you'd like to remove from your ignore list.
Source : help.orkut.com