Here's a short clip describing how to remove students from your WebCT course. It shows how to remove student at a time, as well as a number of students at once.
Note that you should probably download a backup copy of the grade book (select download from the leftmost drop-down box) before doing this. Also, after deleting multiple students, you will want to select "show all students" from the drop-down box, so that you can once again see your current class.
Deleting students from your course (6MB, about 2 minutes long)