Baseball has a "restricted list" that someone refusing to play would be put on. I am not 100% sure, but I believe they are not paid if they are on the restricted list. From wikipedia:
Restricted list - Players who fail to report within 10 days of the season, or who fail to come to a contract with the team prior to that time. Players on this list do not count towards the Reserved List or Active List limits. A player may remain on this list for two consecutive years before he is removed. Often, placing a player on the Restricted List is akin to releasing him without fully relinquishing his rights.