When you install the script, go to the Manage UserScripts form and replace my username with your own in the "included pages" field. When you view your friendspage, there will be a little x by every tag. If you click on it, and then confirm your choice, the page will reload with all entries by that user with that tag removed.
If you want to restore a tag you previously chose to hide, click on the tally of removed entries at the top of the page, and you'll get a menu of tags to restore.
* Ports to other layouts
* Decent error-handling
* Add support for hiding community posts by specified users
My thanks to boutell, whose ljwho userscript provided much of the inspiration and some of the sourcecode for this project.
If you're lucky, all you have to change is the classname in line 42 and the ID in line 50. Use the DOM Inspector (accessable from Tools or right-clicking on your friends page and selecting "Inspect Element") to find if entries have a unique classname, and if the element above them in the hierarchy has an ID. This is easier than it sounds.
Added a version for Expressive