Quick answer - which I have not tried - would be to back up the course and, when going through the series of dialogs, in the Schema settings stage, select only the activities which you really wish to use. Once you restore the course from this backup, these will be the activities which are restored.
Of course, it would be advisable to make a full backup first in case of mishaps. I guess it may be possible to similarly select activities at the restore stage from a full backup.