Do the following:
1) Open a private window (Firefox) or incognito window (Chrome).
2) Go to dominion.games
3) Enter your credentials (name/password) as usual, but do not click "login".
4) Instead, below the login button is a small row of text. Click on "Kick and resign" there.
5) You should get a message "Kick successful". You can now close the private/incognito window and go to your usual tab, where you can now login normally.
6) If you messed up step 3 and reflexively clicked "login", you must close the private/incongnito window and start again with step 1.