I.G.
To block someone on Pinterest:
1. Go to his/her profile.
2. Click the flag icon on the right-hand side of their profile information.
3. On the menu, click Block.
4. Confirm that you really want to block the person.
However.... as long as her board is a public board, that person can still just punch in the URL or google it... I don't even have a pintrest account, but frequent certain public boards all the time.
For pintrest as for any other social media: if you don't want the person you don't like to read it... don't put it online!