You may not want to do this but this is the command
cvs rtag -d -B <tag/branch name> <module name>
-a Clear tag from removed files that would not otherwise be tagged.
-b Make the tag a “branch” tag, allowing concurrent development.
-B Allows -F and -d to disturb branch tags. Use with extreme care.
-d Delete the given tag.
-F Move tag if it already exists.
-f Force a head revision match if tag/date not found.
-l Local directory only, not recursive.
-n No execution of ‘tag program’.
-R Process directories recursively.
-r rev Existing revision/tag.
-D Existing date.