I've tried spamming the logout button. I've closed the browser and returned the next day. I'm still logged on when I open the browser. I've been able to create new posts and change my account details on the session I started yesterday.

clear all cookies from vendetta-online.com in your browser history.

That may hide the symptom, but it won't fix the problem.

If I press the logout button, the session cookie should be invalidated serverside.

If I go to sleep, go to work, and return the next day (while my computer is off the whole time), the session cookie should no longer work.

That same cookie still working 2 days later? Not good.

Internet has been down for a few days, and I've logged on from another computer in between (or a phone actually), but this session remains valid.

Interesting.

I was able to log out properly from the phone btw (Safari, vs Firefox 5).

Heh, those silly devs and their phones! PC needs some lovin.