For user-generated content where you don't have full control on the users' usage of http/https such as in signatures and post attachments, you can add this meta tag in the head to block mixed content. This way, your site will still show the green padlock icon in the address bar.
Code:
<meta http-equiv="Content-Security-Policy" content="block-all-mixed-content" />
If you are on vBCloud, you can add it in the Google Ownership Verification HTML tag in AdminCP, otherwise, you can add it in the head_include template in AdminCP.