I have accidentally added a bit too many words and am really struggling to keep up. Is there a way to cancel some and put them back as I learnt words?

When I do that, I usually ban the extra words, and then unban them as desired.

lechuan's suggestion is good. You can also remove some of the lists that you added, if the words are in separate lists. Later, you can add back those lists and you'll get the words again.

I got into this difficulty when I had to take an extended break from Skritter. I would have liked to be able to withdraw the most recently added x number of words. As it was, I nuked the whole lot though weirdly, now, in 'my lists', the old completed dates remain.