updated 4 Jan 2021
Regardless of your ‘auto play’ setting, Amazon Prime Video displays a “Next Up” box bottom right, with movie artwork, on reaching the credit offset point; because, after spending perhaps more than two hours watching a film, who doesn’t want immediately want to start another?
Adblock rule (last tested Nov 2018):
www.amazon.co.uk##DIV[class="nextUpWrapper"]
Custom stylesheet (last tested Dec 2020):
(in Safari, go to Preferences > Advanced and configure the CSS file to use)
.webPlayerContainer .nextUpWrapper {
display: none;
}
As you can’t specify domain names in a custom stylesheet, you have to narrow it down.