I think the tags idea might be a bit too complex, interface-wise.

I'd prefer an option next to every post in my stream called "fewer posts like this (from this person)". Given enough feedback from me, G+ would automatically start filtering out items for me. Since Gmail accomplishes a similar feat already, this doesn't seem impossible.

However, more practically, I think they could probably just solve my problem by allowing me to mute an entire contact or an entire person. Currently you can "block" individuals, but that's not really what I want - I don't want to block them from seeing me, I just don't want to see their stuff in my main stream.