How to get rid of frequent nesting by pigeons?

I have a duct area adjacent to my bathroom window which is unapproachable - almost 2.5 meters down. Every month pigeons intrude into that space and lay eggs, and their shit generates a filthy smell. My son is also allergic to dust. I can not close the window as that's the only fresh air window feasible for bathroom. Since my flat is at 5th floor and this duct area is approachable only by fire-safety lift, it costs me about 2000 bucks for each cleaning every two months. I can not install a grill or mesh because it's a society controlled area and also may need to used for emergency repairs in my absence. Can anyone help me with any tactic that can scare the pigeons and prevent their entry into the space - some buzzer or other tactic?

I had a problem with pigeons. I have tried artificial eagle, stickers on the glass, balloons with eyes, specific sprays. Nothing helped.
The last and only solution was to put a net over my balcony. It was done by a professional company which put it tightly.

Finally I was able to leave on holidays without worry that pigeon family will create a house in my balcony.