Possible to turn off "Sign Up" link in global site nav

[Verbose]

Hi there,

I am using a site-wide header.ssi file on all pages of my site, including at the top of my vBulletin forum - this header.ssi contains my site logo and global site nav, and also a "Sign Up" link that leads to the vBulletin Sign Up page (/forums/register.php).

Question:
Is is possible to hide the global "Sign Up" link in my header.ssi file after a user has logged into vBulletin using CSS or javascript or something???

The reason being, it is confusing to display this Sign Up link to users who are already signed up!

Thanks!