You can indeed add security on comments on that form. Use LAUA, application security. Select program security, and put your cursor on the desired token (e.g. AR17), then select form security. Put the cursor on the desired form ID (e.g. AR17.1). Press F6 (FC Pair). Use the '+' sign on the numeric keypad to move thru 'authorized form FCs'. You would want to remove the valid pair 'CD' (i.e. form change, line delete). Press enter a bunch of times to save. If you're running portal, you'll need to run IOSCacheRefresh to make security changes active in portal.

I think you are talking about the Delete on the Comments form where you enter the comments.
Security does not affect that. You would have to have a customization of that form created that would prevent anyone from deleting comments.
This might apply throughout your system for all comments, however.

Carol