Its not intuitive...
From the main forum page, click on "all categories" on the top-left (with an arrow next to it) to open the drop-menu there.
Select the category you want to ignore, this will filter it to just those. (Ordinarily, this is just useful as a filter...)
Once you've filtered to this category, on the top-right side there is a button with a circle in it like we get in individual topics. Click on it and select 'Muted' to never see or hear from that topic again.