If you know the previous owner they can simply log into their account and transfer the registration to you, otherwise you can make a support ticket on the store.dmxsoft.com website under your account and tell them to move it to you.
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 .