Yes, it is possible to block specific users from appearing on your friends page. This requires advanced customization and the use of a theme layer.
Where can I learn more about advanced S2 customization?
More information about advanced customization can be found in the above referenced FAQ. In particular, this FAQ links to the s2howto memories, in which you will find a number of tutorials for "Hiding comments/entries". To use one of these tutorials, you will either need to switch to the appropriate layout or use the tutorial as a guide for writing your own code.
Please note that LiveJournal Support volunteers are unable to provide personalized customization assistance. If you require further help, you may wish to use the following FAQ for ways to find a community that offers additional help with journal customization [http://www.livejournal.com/support/faqbrowse.bml?faqid=85&view=full].
If you choose to share this information with other users, please note that advanced customization is a Paid Account feature. Information on how to buy a Paid Account is available in [http://www.livejournal.com/support/faqbrowse.bml?faqid=21&view=full].
And if that's too techy for you, at least using the "block images from this server" in Firefox works. Cheers!
ETA: disabling further comments > take it to e-mail or personal blogs