This is a limitation in place from Twitter's API, unfortunately. They only allow interaction with the most recent 3200 tweets at a time. However, if you clear some out, you'll be able to work your way back further.
You can cancel (or re-activate) your account any time from the My Account page. No hoops to jump through. No questions asked. Your membership will run until the end of your current period and you won't be charged again.