Errr... I've just been experimenting.. if anyone's interested, you can hide a poster using a chrome extension and the right XPath expression...
I just blocked Helen (temporarily!) using the following:
'CustomBlocker' chrome extension (just search the Chrome store)
then, when on mumsnet, click the funnel icon in the top right.
Edit the target site, putting a star where you want a wildcard - so I've only blocked in FWR, so my URL becomes:
www.mumsnet.com/Talk/womens_rights/*
Then, in the 'Elements to Hide' section, click 'XPath' and enter the following:
//DIV/SPAN[SPAN='HelenDenver']/../..
(substituting the nick you want to block - Sorry Helen! You were just in the middle of my screen when I tried this!)
click save, and now, when you load an FWR discussion, it'll do a little hiccup and hide anyone you've added a rule for...
to add more rules just click 'new rule for this site' and do the same.
I promise I've removed my Helen block now 