If a sub feels uneasy about doing a certain activity/task, he or she should not feel obliged in any way to do said activity/task. I would consider the activity/task a limit for that sub.
If the sub needed to be punished, I personally do not feel that a limit should be used. Limits are there and should be respected.
If a dom ended a relationship because of refusing to do something which was a limit, I believe the dom would be acting unreasonably.
The relationship between a dom and sub is not governed by one set of rules, they are set by the parties involved. A submissive can have as much say as the 'rules' of the relationship dictate.