If you have the patience to manually blacklist URLs, you can do this pretty easily with a regex-based site blocker. (My favorite for Chrome is Simple Blocker, which doesn't document that it uses regexes but in fact does: https://chrome.google.com/webstore/detail/simple-blocker/akf...). For example, the following blacklist regex patterns block the Reddit front page and the main page of any subreddits and multireddits, but still let you navigate to individual threads:

    https://(www|np).reddit.com/?$
    https://(www|np).reddit.com/r/[^/]+/?$
    https://(www|np).reddit.com/user/[^/]+/m/[^/]+/?$