This tool will allow you to mass unfavorite posts on e621. You can use it to unfavorite all posts, or ones that pass a filter.
The filter syntax is much like my advanced search syntax, however it does not have any metatag support. It does, however, have full tag grouping, OR and NOT capability. Read the above post for more details and examples.
The program must be ran with node.js. I recommend running it dry first if you are using the filter to make sure nothing looks wrong and that the filter worked properly. After that just let it run. It will do one unfavorite every 800ms if the post passes the filter.
To use this you will need to download node.js and then run the program.