We find it would be very useful to be able to exclude posts with certain tags in a search. Now this is supported in BibSonomy via the sys:not system tag.
You can use this tag in the tag search as a prefix for any tag. This causes the system to exclude every post which is tagged with the tag that follows the sys:not operator.
For example some of my colleagues like to tag publications which they have already read with a tag like “read”.
In combination with the sys:not system tag, they are now able to find all publications which they haven't tagged as read yet by querying for sys:not:read.
Hope you'll like this feature and happy tagging!