I haven't been able to log onto www.spamarrest.com for months. I can
go to any other wegsite with no problem. I can't be the only one with
the issue, but I don't see any chat on the net about others with this
problem. And there's no way to contact the company other than via
their website. What could be the cause?

can you access the website from another computer? if so then the
problem is local on your own system.
try going to
 http://66.150.163.163/
or 
 http://www.spamarest.com/ (notice it's missing an r)

since these are alternate address to connect to it. 

if those don't work try using a proxy service like
http://www.the-cloak.com
which will go to the site for you.