1) someone who is trapped in a conversation against their will. often they will try to change the subject, but no matter how many times they do, the conversation always moves back to the original subject. Sometimes, it is avoidable, but there are many scenarios where one cannot escape - eg, in college/school where one is sat at the same desk as a conversation jailer.
2) to hold someone conversation hostage
1) Jemma was forced to listen to Daisy talk about her dad and Burger King for an hour because she had no means of escape. Boy did she get held conversation hostage!
2) man, that Daisy held Jemma conversation hostage for an hour talking about Burger King! She's such a conversation jailer!