This response was provided by Darryl Coates, Illinois Department of Natural Resources, District Wildlife Biologist.
Squirrels become a nuisance when chewing on siding, doors, door trim, deck wood or wood patio furniture. Sometimes, squirrels acquire a taste for the soft woods. Most of the time, it is trees and shrubs that are effected. On occasion, tree squirrels will select siding, trim and doors. When this happens, I recommend that you begin by using taste repellents and exclusion techniques. There are a couple of taste repellents that can be used to discourage squirrel chewing. The first is Capsaicin. This product is made from red pepper (a natural deterrent) and the second product is Ro-Pel. Both are ready-to-use (RTU) products. Exclusion techniques require that the wood be cover with metal or hardware cloth. If taste repellents and exclusion techniques fail to resolve the problem, removal of squirrels by trapping is recommended. Removal requires a nuisance wildlife permit issued by your DNR Wildlife Biologist. You can learn more on the "How to Solve a Problem with Wildlife at Your Home Article: Hiring a Nuisance Wildlife Control Operator Vs. Do-it Yourself" page on this web site. If you decide to trap the squirrel or to hire a Nuisance Wildlife Control Operator go to the "Find a Wildlife Professional"page. Contact your local IDNR Biologist by Right clicking on 'Find Your District Wildlife Biologist' should you chose to Do-it Yourself and to receive a nuisance permit. Right click - 'Find Operators by County or Region' to locate a Nuisance Wildlife Control Operator in your area should you chose to hire someone to come in and remove the squirrel(s) for you. For more information on squirrels and damage prevention and control measures look under "Directory of Wildlife", 'Mammals', then select 'Squirrels' and you will see additional recommendations. To locate taste repellents contact your local Tractor Supply or Farm Supply Stores or a Pesticide Dealer. Thanks for contacting us.