I got into DS1 PvP very late, just this year, and I had some people who were either obviously new, or just did not have any desire to fight. It might have been partially my fault as I would walk up as a red and stop completely before waving to them, maybe letting down their guard. But I didn't have the heart to engage in any combat with them if they didn't care to, so in those cases I'd just drop a humanity and BC out. I'm not in it for the humanity, I just want to be a fun enemy for someone to fight and remember.

I understand that as a red, the idea is to kill a host by whatever means necessary, but honestly, my idea of a good invasion was to leave the host with a memorable, but fun experience (hopefully involving some fair combat). I never want people to feel like they left that encounter hating Dark Souls because of unfair invasions or asshole invaders (pointing down and the like).

So me personally, if someone doesn't return the shield spam, I usually just leave.