Moodle 1.5 supported three different section formats: weeks, topics and social. Moodle 1.6 added LAMS, SCORM and Weekcss section format choices to that list. The last three are a little more connected to the rest of the code (and hence, less "pluggable") but it is still quite easy to add new ones.
An administrator wishing to remove installed formats may do so by removing the format plugin from the installation. That is: remove the directory containing the format for the moodle/course/format directory. You probably should not remove any of the three core formats, or formats that are in use on the site.