Squirrel Repellent Plants

I participate in a collective garden which is located in a lovely church yard. We grow organic vegetables, herbs and flowers for the participating gardeners and our surplus is donated to our local food bank. We share this location with a large population of squirrels who are well fed by the residents of a retirement home which is also on the site. The squirrels dig, nibble, eat and make quite a mess in the garden and the idea is to keep them away.
Can you recommend a plant or method to repel these squirrels? Would the piss-off plant work? If so, can you tell whether this is sold only as a plant or can we purchase seeds? Any other suggestions would be appreciated.

I have not tried it for this, but since it repels rabbits it might do the same for squirrels, since both are rodents. They need to be planted three feet (one meter) apart around the garden and brought indoors for the winter.
The plant is a hybrid and would not come true from seed . It must be propagated by cuttings and this is protected by patent.
Hot chilis are also repellant. Make a spray by blending some with water and spread it around with a watering can. It needs to be reapplied after a rain.
A three foot (one meter) wide border of nasturtiums around the garden is supposed to work as a rodent-repellant. It would look marvelous as well! Any tree would have to be circled as well to make the squirrels think twice about coming down.