Monitor the content filter and fix incorrectly coded filter hooks (that send text to the webpage instead of returning it, as all filter hooks must).
A notice is sent to the PHP error log when webpage output is detected:
[01-Oct-2017 01:48:28 UTC] Block Filter Output: The "ClassName::echoText" hook with priority 10 in the "the_content" filter has incorrectly sent output to the webpage. All WordPress filter hooks must return their text, not send it to the webpage output. Please contact the author of that filter hook and report this issue as a coding error. Incorrect webpage output:
<strong>Some output that should have been returned instead of echo'ed.</strong>