You can not set the description field to be readonly. However, you can setup Mantis in a way to allow them to report issues, but not modify them once they are created. They can only add notes to them. You can then modify the issue as you see fit.

Unfortunately, we like the ability for all of our users to "update" change defect information as all of our user base has the potential to be assigned a defect. Is it a possible to include a read-only flag in config file for future releases?

Thanks,
Ann