I registered just to post this, then realised that for some reason, Deskthority is no longer returning 403 Forbidden errors now.
Anyway, if it does it again, there are 2 possible workarounds :
a. Manually edit a link that shows you a 403 Forbidden error
If you look at the URL for one of these links, you'll see that it looks like this -
hxxp://deskthority.net/viewtopic.php?t=<some number>&sid=<some other number>
Delete the bit that starts with &sid all the way to the end of the line and hit enter. The page should now load.
b. Click on the Spy section on the top left of the screen, then go into the latest posts from there
For some reason, when you click on any links in the Spy section, they don't contain the &sid=<some number> indicated above, so the links load