To enter, you'll need a FREE SPIRIT account, and have to defy grandma's advice to not say anything at all if you can't say anything nice.
Instead, you'll need to send a hate message of no more than 140 characters about Spirit, or another airline, to this website. Though if you're one of the rare people who like Spirit, it'll let you say something nice, too.
Within 10 days of entering, you'll get 8,000 miles in your FREE SPIRIT account, which you can also redeem with existing points, if you have any.
Just one caveat: The minimum number of miles needed to pay for a one-way Spirit flight is 10,000. Unless you're a Spirit World MasterCard holder (which you're probably not), you'd need an additional 2,000 miles to actually get anywhere. Anyone else will have to cough up the additional miles and pay for the return flight.