Hmmm. 
I did think it was because they were a Category Moderator, but I’ve just checked your /about page and it doesn’t look like you have any - https://community.smartthings.com/about
Just to check, does this user have full Moderator (or Admin) abilities? You can check from their admin user page in the Permissions section:
There should not be a way for a TL4 to delete a topic without being granted extra moderation permissions.
I can help you add that CSS snippet to a custom theme component, but it may be worth tracking down exactly what’s happening first. 