My courtyard, bounded by buildings on three sides and a wall on the fourth, seems to attract squirrels. They dig in any fresh dirt (think flower seedlings or new plants), chew on the furniture and sit on cushions chewing nuts (leaving their deposits).

I've tried the "Repels All" animal urine spray. To me, it certainly stinks but the squirrels will happily sit on a saturated fence top oblivious to the stench. Squirrels will stand on an ultrasonic repeller--they can no more seem to hear it than I. 

A "Yard Enforcer" motion detecting water sprayer will wet the furniture, me, plants; most everything except the very agile water avoiding squirrels. 

I've run three strands of taught fishing line atop the wall. Squirrels come through that like a man crawling through a barbed wire fence.

Don't even mention the plastic owl. I've set that out and moved it twice daily. If it gets in the squirrel's way (like on the arm of a chair), they'll knock it off so they can sit there and chew away.

Someone told me that wind powered pinwheels would deter the critters and I haven't tried that (figuring that I don't get enough wind in the enclosed courtyard to generate much movement).

I happen to have a "magnetic spinning top," one of those battery powered "perpetual motion looking" gadgets that are available for a few dollars. I wanted to use it in Santa's Shop, but it is annoyingly noisy (Santa's Shop is in a business window in December and the noise would be disturbing for the workers). I tried operating the thing in my workroom, but the noise was too annoying for me after a few days. 

"Why not bug the squirrels?" I questioned, so I set the thing out on a table in the courtyard during daylight hours (7 to 7). It's been out there for a week and I've had zero squirrel visits. Maybe this is random, maybe the noise, maybe the flashy movement--just thought I would share.

UPDATE  4/21/19

Still no squirrels when the top is running on the table in the center of the courtyard. Last Friday, a storm front was expected, so I didn't put the top out. Within four hours, squirrels were in the courtyard. I chased the squirrels out and placed the top on a sheltered porch about 20 feet away. Squirrels came in again.

The next day, I placed the top on the table in the center of the courtyard--no squirrels.

I'm speculating that it's not the noise, but the movement of the shiny top (visible from most squirrel entry ports) that has the effect.

Will it wear off?  I'll update later.

UPDATE 4/27/19

Still no squirrels in the courtyard . . . well, I saw one skulking around the far edge, so I enhanced my deterrent arsenal.

I took a few "dollar store solar movers" and added bits of aluminum duct tape to random spots.

The pink flamingo also rotates on a solar powered base.

Movement of the shiny duct tape seems to keep the squirrels away. Why?  I have no idea--but they seem to avoid the devices.