Make a user-permission for allowing thread authors to delete replies to their post.
Another for banning users from a specific thread would be great, too.
I think I understand where you are coming from but your proposals suggest that you are allowing your users to post inappropriately and then need to remove individual inappropriate posts. This is the approach used by mail lists. However, forum software is not designed for low traffic sites where you can be expected to monitor every post. Nabble, and every other forum package I know, is built on the basis that users must gain the administrator's trust and are then granted a greater level of access.
This means you need to create a number of sub-forums where only your trusted users may post. You then use User Groups and Permission facilities to limit posting rights to those areas to those you trust.
I have a section on Controlling Access to a forum on my GregHelp site. See if the information there helps. If it doesn't ask your questions here as that will give me a chance to improve my guidance.
Just a Volunteer Nabble Helper - because the nice folk at Nabble have helped me! GregHelp - Building a set of answers to Nabble FAQs.