The guest is allowed to inquire because anyone can send a inquiry. Let's say he was inquiring about dates 6 months from now and wanted to book a 2 week stay- that wouldn't likely trigger the algorithm. An inquiry is just a way for guests to ask questions, so anyone is allowed to do that.
If it's an Inquiry, there is no need to decline it- just messaging back to an inquiry is sufficient. If it's a booking request, you do have to either accept or decline within 24 hours.
I'm not sure what you mean by asking if you were to accept a blocked inquiry, what would happen. Even if you were to approve it, it wouldn't go through if Airbnb has blocked it. If you found some other way around it, to accept the booking, I think that Airbnb wouldn't be supportive if the guest wreaked havoc. It's hard enough to get them take appropriate action on a normal booking if the guest causes damage- if they already blocked the booking, I think they'd tell you, sorry, you cooked your own goose.
And it isn't the guest himself that's suspicious- it's the combination of booking factors which trigger that block. If he were trying to book a shared home listing, it wouldn't get blocked, only entire home listings.
I agree- "coming for a job interview" over a weekend, with his girlfriend, when he lives 10 miles away, doesn't sound very likely.
@Megan601