If you can directly access the areas they're nesting in, a few Bounce (or other similalry STRONG scented) dryer sheets shoved behind the wall will deter them for a few weeks at a time. I do maintenance on a heritage building and for several years we managed them pretty successfully by poking a hole through the drywall everywhere we could hear them up against the wall and poking in a few dryer sheets, install a plastic access hatch to save on drywall repairs, repeat every two or three weeks. We only ran into problems when they'd managed to chew their way in from the outside to an area we couldn't reach, eventually had to call in pest control to put one-way squirrel gates where they'd chewed into the eaves after I blocked their previous highway...
If you can directly access the areas they're nesting in, a few Bounce (or other similalry STRONG scented) dryer sheets shoved behind the wall will deter them for a few weeks at a time. I do maintenance on a heritage building and for several years we managed them pretty successfully by poking a hole through the drywall everywhere we could hear them up against the wall and poking in a few dryer sheets, install a plastic access hatch to save on drywall repairs, repeat every two or three weeks. We only ran into problems when they'd managed to chew their way in from the outside to an area we couldn't reach, eventually had to call in pest control to put one-way squirrel gates where they'd chewed into the eaves after I blocked their previous highway...