Having run into this problem before I have found that the easiest and most secure way to handle this is to have your site hosted with HTTPS which these days isn't anywhere near as expensive as it once was. With having your site hosted through HTTPS no referrer header will be passed to the linked site. The difficulty with manually specifying the
utm_source parameter is that I have found several ways to bypass this and prevent it from causing issues with GA, mainly from sites doing exactly what you are trying, blocking referrer, but in doing so it threw off my analytics reports. By using HTTPS you are not artificially affecting the linked sites analytical reports as it is known to be from an external link but the link is unknown, instead of artificially saying the link was an internal link which is false.
There is a move to make a standard meta tag that would do exactly this but the support for the meta tag is not yet standard across all browsers. By using
<meta name="referrer" content="never">. The meta referrer tag is currently supported on Firefox, Chrome, Opera, Android 5.6 browser, and Chrome for Android, with partial support for...
- iOS Safari - Supports an older draft version of the standard with never, always, origin, default.
- Safari - Supports an older draft version of the standard with never, always, origin, default.
- Microsoft Edge - Supports an older draft version of the standard with never, always, origin, default.
Based on this your best option would appear to be to use the meta if you are concerned about HTTPS to HTTPS referrer's. There is no support for meta referrer in IE but that is due to Microsoft no longer actively developing IE in favour instead developing Edge.