The Winter can only be stopped by completing the Dear Santa Mission. To complete the mission, you need all Santa's Reindeer. You can obtain them by spotting Santa when he's flying over your city, or by playing the Reindeer Games in your own of one of your friend's cities.
NOTE: If you want to collect more Reindeer (e.g. to complete more missions or to help your friends) after you completed the Dear Santa mission, you need to re-trigger the Winter event.