This is why I 1) only hang around coffee shops with big seating areas, 2) pick the smallest table near a power plug, 3) try to buy something every two hours, be it water, coffee, sandwiches or muffins, to "simulate" turnaround as much as possible, and 4) evangelize for my local on G+, TripAdvisor etc.
I'm probably a bit over-sensitive because I come from a country (Italy) where waiters and owners will basically tell you to sod off if you hang around a table more than one hour... which is probably why most cafes there never offered wifi in the first place.
> I come from a country (Italy) where waiters and owners will basically tell you to sod off if you hang around a table more than one hour
Of course, that would also be a perfectly reasonable solution. It has the big advantage of being straightforward, directly addressing the actual problem without inconveniencing non-campers), and letting the customer know what the issue is, and it seems much more likely to work than any of these technical tweaks... (after all, you can easily use a laptop without wifi in many cases, or with a cellular modem, and some campers aren't using a computer at all)
Ideally they'd also apply such a policy thoughtfully, e.g., being more lax when the cafe is uncrowded, and letting you stay around longer if you were buying enough stuff. [After all, you're essentially renting space, and the appropriate price shifts up and down with demand]
Around here they explicitly put up signs saying they may ask you to leave if it's crowded and you've been there for a long time, but I've never, ever, seen that policy actually applied (and some people, mainly students studying, sit there for ages and ages nursing small coffee).
Personally, I'd also like it if they also gave one the option of straightforwardly renting space... sometimes I like to sit in a cafe, but don't really feel like drinking or eating anything...
I wonder if this is an American issue. I'd like to think that in the UK we're taught to be polite and that sitting at a table for hours on a single coffee would be rude.
I'm probably a bit over-sensitive because I come from a country (Italy) where waiters and owners will basically tell you to sod off if you hang around a table more than one hour... which is probably why most cafes there never offered wifi in the first place.