How to add write-protection to flash drives?

I volunteer with a educational not-for-profit organization. We are planning to start providing flash drives with content instead of paper-filled binders at our education events. I would like to write-protect the drives before distributing them to attendees so that they cannot add unapproved content or wipe the drive & convert it to personal use. I’ve found lots of web help to remove write-protection – not much about how to enable it. Any suggestions?