There is an administrative permission Change Authorship that controls the ability for users to change the author on posts. Note that this is an administrative permission for good reason: it gives users the ability to post "on the behalf of" other users (aka masquerading as other users). As such, it's not possible to grant this permission to all members on your site. The permission can be granted to users you trust to use the functionality judiciously.
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 .