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ETA: This post is outdated. AO3 coders have implemented full exclusion filters. Yay!
Since someone just asked, I thought I'd post this more broadly.
Blocking categories of tags on AO3 is actually quite easy. You don't even need to know CSS, because other people have already written the code. You can block a single category of tags or all tag categories very easily. With only a couple minutes' work, you can also block any combination of tag categories.
If you only want to block one set of tags:
1. Go to http://archiveofourown.org/skins
2. Search the page for "Blurbling Hide".

3. Choose the category you want to block and press "Use" in that skin's box. (For warnings, you can use "Hide Warnings" or "Faint Warnings" for different effects.)
If you want to block multiple categories of tags, do steps 1 and 2 above. Then you can either use the skins called "Hide nearly everything" or "Hide tags", or you can make your own easy cut and paste CSS skin:
3. In a separate window, open http://archiveofourown.org/skins/new
4. Give your skin a title and description. You can skip the two steps below the description.
5. Back to the Blurbling skins: click on the title of a skin that hides something you don't want to see, such as "Blurbling Hide datetime".
6. A new page will open. Copy and paste everything from the /* to the } (which is all the text below CSS).

7. Repeat steps 5 & 6 for each tag type until satisfied, then press "Submit" at the buttom of your custom skin page. (Note: the order in which you paste items doesn't matter for this particular goal.)
8. On the next page, press the "Use" button (bottom right).
9. Go look at the archive and make sure everything's showing (or not) the way you want it to.
Disclaimer: This worked for me. If it doesn't work for you, upload a screencap of your CSS and I'll try to help, but I'm not a CSS guru.
Note: You'll still see the archive icon that indicates whether or not a work has warnings, you just won't see what they are.
Since someone just asked, I thought I'd post this more broadly.
Blocking categories of tags on AO3 is actually quite easy. You don't even need to know CSS, because other people have already written the code. You can block a single category of tags or all tag categories very easily. With only a couple minutes' work, you can also block any combination of tag categories.
If you only want to block one set of tags:
1. Go to http://archiveofourown.org/skins
2. Search the page for "Blurbling Hide".

3. Choose the category you want to block and press "Use" in that skin's box. (For warnings, you can use "Hide Warnings" or "Faint Warnings" for different effects.)
If you want to block multiple categories of tags, do steps 1 and 2 above. Then you can either use the skins called "Hide nearly everything" or "Hide tags", or you can make your own easy cut and paste CSS skin:
3. In a separate window, open http://archiveofourown.org/skins/new
4. Give your skin a title and description. You can skip the two steps below the description.
5. Back to the Blurbling skins: click on the title of a skin that hides something you don't want to see, such as "Blurbling Hide datetime".
6. A new page will open. Copy and paste everything from the /* to the } (which is all the text below CSS).

7. Repeat steps 5 & 6 for each tag type until satisfied, then press "Submit" at the buttom of your custom skin page. (Note: the order in which you paste items doesn't matter for this particular goal.)
8. On the next page, press the "Use" button (bottom right).
9. Go look at the archive and make sure everything's showing (or not) the way you want it to.
Disclaimer: This worked for me. If it doesn't work for you, upload a screencap of your CSS and I'll try to help, but I'm not a CSS guru.
Note: You'll still see the archive icon that indicates whether or not a work has warnings, you just won't see what they are.