Hi,
There are 2 possible workarounds, the easiest might be to just hide the li element with css, you'd have to view the source and find out the id on the li element to make a css rule to hide it, then it could only be reached by direct url.
By default that link is only shown in the master site, so if you had a multi site installation it would not be shown in the child sites. I imagine your current site is already the master site but you could create a new site like admin.yourdonain.com then in the database change the new site to IsServerAdminSite = 1 in the new site and 0 in the current site in the mp_Sites table.
Hope that helps,
Joe