Protect your post or your link by using [hidepost] and [/hidepost] between the protected content.
A WordPress plugin that protect a part of your blog posts (maybe links, images or texts), that only registered and logged in members can see it. Simply put your content to be hidden with [hidepost] and [/hidepost].
If you want to protect your post to some user with low level, you can use [hidepost=level][/hidepost]. User level can be found on Codex page.