g1smd - 1:25 am on Mar 14, 2012 (gmt 0)
If you block access via robots.txt you will just have tens of thousands of errors in that part of the report instead.
The alternative URLs should have been served with the rel="canonical" attribute in their <head> section, pointing to the standard URL for each page.
The alternative is to alter the PHP script that generates the page to instead no longer show the font sizer button/link when it is Google requesting the normal page.
If there are no links pointing to the alternative URLs and those alternative pages no longer exist, the errors will clear themselves from the list within a few months.