For some reason you want to delete all your old tweets from your Twitter account to make it look good or you think are not worth to be there in your account. Also I think there are many reasons because of which you may want to delete all your tweets. So let us see some online services that will help us delete tweets automatically and quickly.
Also see Twitter allowing users to download all their tweets.
Tweet Delete: As the name itself suggests Tweet Delete wipes off tweets from your Twitter account. It can delete up to 3,200 tweets at once. If you have more than 3,200 tweets to delete then go for this service.It allows you to choose the timeframe of one week, two weeks, one, two, three or months or a whole year.
Delete all my Tweets: This tool can delete tweets up to 1000 in one go. To delete more repeat the step again.
Tweet Eraser: This service provides features like filtering tweets based on hash tags, text strings and dates.
Tweet Wipe: Wipes out all tweets in one go.
Delete Multiple Tweets: Select and delete multiple tweets from your Twitter account.
Also see Download high quality Twitter header images.
Wipe out all tweets from your iPhone:
Have an iPhone, want to wipe out Twitter account from your iPhone Tweeticide does the job good. Tweeticide shows your account name and number of tweets when you login. It gives you big button, delete all tweets, click it and your tweets are gone.