The rule is that when a creature is subject to a mandatory action, such as Taunt or Bloodthirsty, they must perform that action IF ABLE TO. If they are unable to perform that action, then they are not subject to any restriction and can perform any action they wish. This is on page 11 of the Rules Supplement.
In your example, they are unable to fulfill the Taunt requirement due to being restrained, and thus may perform any other action they wish. Unless they have a ranged attack, in which case they'd be required to shoot at Thorg while they sink.