Others have virtually quit, usually by opting out of tasks and taking major penalities, dropping them to last without hope of catching up. Nick and Vicki did it in Hong Kong (race #17); Nick opted to sleep through the task and, eventually, Vicki decided they'd take the penalty. With an extra four hours on their start time for the next leg, they were eliminated after completing their Speed Bump in South Korea. No chance save total closure of flights.
Come to think of it, it happened once more in TAR15 (in the leg before Maria and Tiffany). Team involved: Mika and Canaan. Mika refused to slide down the waterslide at Atlantis, The Palms in the UAE. As such, they were eliminated...because of her decision not to complete the slide. She did virtually quit.