If you’re on a desktop (or other large screen), click on the user name to go to their user page and there’s a block button in the sidebar on the right. If you’re on a mobile device (or other small screen), go to their user page and the block button should be prominent on the right of the “follow” button.
I do this, too, and I’ve been wishing there were a setting I could set where kbin would just auto-hide content submitted by accounts that have been blocked by at least X other accounts.