Maybe a one-time installation of a grate screwed down to the 'floor' of the ledge a few inches off the ground - would be snug & tight, still allow for visits but not nesting? 
The primary concern is safety, how do we maintain it and keep peregrines (and other birds), pedestrians, vehicles, property (our hotel and other buildings) safe? We have had a number of ideas that would keep the birds off the ledge for nesting but still give them access to the ledge for other activities. Problem is we can't just throw something up there and screw it down as appealing as that sounds
... that will damage the roof and cause leaks into the meeting rooms which is an unnecessary cost and inconvenience for the hotel that hosts our birds. We check and do any repairs/cleaning/replacing of our nestboxes twice a year, spring and fall. And what about when the hotel has to do maintenance or window cleaning, what won't interfere with the work they need to do? And when it rusts because of our wet and snowy weather? or the points where it gets attached are compromised because we've drilled into it concrete (think freeze-and-thaw cycle) - then we have to redo it all again and we damaged the building unnecessarily and perhaps caused structural damage as well. And how much will it cost to have someone working off a lift - no power, can't work when its at all windy or wet or cold for safety reasons, you can see it isn't a quick fix.
Basically, we are thinking about what we might be able to do. Then we will look to see if we can do it safely. Then we will look at raising the money to do it. Then we will look at how best to do it. And then we'll do it. Or not because its too dangerous, too expensive, the peregrines have moved (or become extirpated) or, or, or. Technically we don't need to do anything and some would say that we shouldn't do anything because they are wild nesting peregrines who are choosing to nest there and "there" is no different from a wild nest on a cliff face (which is very true). Let nature take it's course. And that is just as reasonable as going through all sorts of hoops to block off access to a ledge that has been used 5 times (I believe) in 30+ years. So there is an argument both ways. In the meantime, I keep looking at options ...
