There's two ways to do this. You can do it manually from the explorer on the dashboard. Search for the feed, select an activity and press the truncate feed button. This is the easiest way to do this if manually doing this is sufficient.
It's also possible like you found to use the delete API endpoint to do it programmatically. This endpoint is not built in to most clients, including the Java client. The URL and HTTP verb that you used should indeed work.
From what I can tell from your headers and the response it seems like you are having an issue with supplying a correct signature. The easiest way to do this correctly is to use the built in methods in the library you're using to generate them. I'm not an expert in the Java library, but it seems like these methods are inside the StreamRepoUtils class.