This response has been taken from Crowdflower's FAQs:
The system says that I am banned/flagged etc, why is that?
This can happen for a number of reasons, if you have submitted more wrong answers than our accuracy threshold will allow multiple times, it can sometimes result in banning/flagging from a single job or from our jobs in general depending on how many times you have been banned from individual jobs. Sometimes our security system will automatically flag workers for submitting a suspicious amount of work in a short amount of time, if you are wrongly flagged it is very easy to de-flag you, so feel free to send us an email if this comes up. You can contact us at firstname.lastname@example.org.
Keywords: what is crowdflower?, crowd flower, Memo Tasks, memotasks