Is there any way to get rid of the stupid action bar that pops up on the right of every message that one scrolls past? It is annoying because it is both visually distracting and often blocks the right hand side of previous messages.

there's no slack option to do that, but if it's important to you you can use a browser plugin to modify the CSS

span.c-message_actions__container or something; just set it to visibility:invisible; (although, some of the buttons in there are occasionally useful)

like this plugin for chrome https://chrome.google.com/webstore/detail/stylebot/oiaejidbmkiecgbjeifoejpgmdaleoha?hl=en-US