Sorry for bumping an old topic. I was just looking into this issue. I agree that if a warned user removes themselves from the warning PM, that indicates that the user has seen the warning. It would probably be technically possible to prevent users from removing themselves from warning PMs, but it doesn’t seem to be an issue that comes up very often, so implementing that is probably not something that needs to be prioritized.
There is a minor UI issue when a user removes themselves from a warning though. The warning count is still displayed for staff on the user’s profile page, but clicking the warnings link takes you to /u/<username>/messages/warnings and displays the “You don’t have any messages” text:
The warning PM still exists, but it’s no longer in the user’s warnings inbox. The UserWarning record still exists in the database. That entry contains the user_id of the warned user and the topic_id of the warning PM. It seems that it would be possible to use those records to link to previous warnings that have been sent to the user instead of linking to /u/<username>/messages/warnings.