Found the answer myself :) So for anyone who stumbles upon this: no, there isn't a way to prevent users from updating issues that have been shared with them, even when they're closed. Kirsten Heikkinen Trek Bicycle Corporation
Kristen, We had this issue as well. I added a custom status that says 'Converted to Project' so it displays for our requestors to see. Then, we train them that if your request says converted to project no updates will carry over from the request any longer. Christina Jarosz Ascensus
Thanks Christina, I didn't think of that. This was brought to my attention by a user who isn't actually converting their issues, but this is still nice to know! Kirsten Heikkinen Trek Bicycle Corporation