I've just had 30 points removed because of 'USER REMOVED'. This is the only explanation I can access. What does 'User Removed' minus ten points mean?
It means that someone who previously voted for you had their account deleted, and their votes were removed at the same time as their account. This can either be at the person's own request, or imposed by Stack Exchange for violating the terms of service on this or another SE site.
If you look at your inbox on the site's top bar, you should see a number of entries saying "User was removed" that add up to 30 points. For example, 3 x -10 for answer upvotes that were removed, 5 x -6 for question upvotes, or some combination of the two.
Clicking on one of those entries in the inbox, will bring you to the "reputation" page in your profile; there will be a "learn more" link which leads to a page in the help center which explains it: