when in this situation, I just try to ignore the other spectators and focus on my primary audience.
I think this is a bad approach. Firstly, if you ignore a new spectator they will not feel included in the audience, therefore they are subconciously free from paying attention to what you want them to. Also, if you alienate them from the group and they spot something, they are going to be even more inclined to tell you about it.
Personally, if I find myself in this situation I always do my best to stop the trick and properly introduce myself to the new person. If that's not possible I will always make eye contact and smile. This immediately gets them involved and now they're just another member of the audience.
If they've positioned themselves at an awkward angle I'll try to re position myself "so that they can see everything" (in actual fact I'm shifting position for the exact opposite reason).
Having said that, there are very few parts of my routines which are angle sensitive, for this very reason. If this is an increasing problem, I suggest you reassess the material you're using.