Back in the day it was near impossible, but I think a lot of that had to do with the shady founder and the company's aggressive attitude under him and he has since left.
But I always try to use a debit card just in case. Then when they try to recharge an empty card...jokes on you Flo! I actually got like a year of free Flo this way because I technically never cancelled and they kept trying to charge an empty card. Karma!