There is a brand new ‘User Group Membership through Badge’ script for the automation plugin that may be suitable - though rather than adding them to a Trust Level group directly I think I would consider adding them to a custom group which granted a particular trust level (that way they shouldn’t be downgraded when the trust levels are periodically recalculated).
For this, I’m thinking Unlisting the topic is likely the best route. Unlisting does still send out notifications though for anyone Watching/Watching First Post a category, so you may also want to consider creating a brand new subcategory to put it in which nobody will have had a chance to start Watching yet.