This depends a bit on your internet browser, but if you clear history and cookies it should wipe the save data from the game, resetting it. This will also clear any other save data from other online games as well, so keep that in mind. I should have added a full reset button, sorry about that.
Another option would be to use an incognito or private browsing window to play the game, which will give you a fresh start. This doesn't clear any other save data, but the downside is that you can't save your progress if you want to come back afterwards.