I don't know your purpose for using Moodle, however, as a former administrator in both K-12 and higher ed, I want to offer the following cautions, particularly if your courses are used for any kind of credit such as professional growth or continuing education units, certification/recertification, test prep, etc.
I can understand wanting to keep your server load low, however, if there is any potential that someone can challenge you/your company/your institution about the course quality, the work the individual did or did not do, the quality of the work, etc., you may find yourself in a position where you need to produce evidence.
That said, my recommendation would be to do a course backup with all student data and logs and archive it with a meaningful name and a destroy date (eg., Course 1 - destroy March 31 2015). If certification/recertification is involved, I'd save for at least 5 years -- maybe 7 or 10 to cover all possible timelines.
Once the backup is archived, I'd then remove the course and data from the active server.
Hope this helps.