Door to door solicitation is a a common problem in neighborhoods and one that comes up periodically in mine.
Municipalities can pass rules governing solicitations but the Supreme Court has upheld the constitutional right of missionaries, politicians, and others to knock on doors without first getting permission from local authorities. Go here from some references on this ruling: 1 2 3

The Berkeley Police Department offers this advice on handling Door to Door solicitations:

If you feel that the person who has come to your door is trying to sell or solicit anything under false pretences, please immediately telephone the police. If the solicitor threatens, intimidates you or refuses to leave, please dial 911 for immediate service.

We also recommend that you acknowledge anyone knocking at your door or ringing the door bell.
After not answering a knock at the door, residents occasionally find that they were victimized by the person who had just knocked at their door. You should never open the door to someone you do not know, but always acknowledge that you are home. If you prefer, go to the door with the phone in your hand.

None of this is intended to say that all persons going door to door are doing so illegally or that they are criminals, although historically, there have been many persons soliciting under false pretences. You are the only who can decide if you feel that the solicitation is suspicious. If you are not certain, call the police for assistance. You have the right to ask them to leave printed materials so that you can check into the business or organization. Then you are able to do your own research and if you decide, you may send them a check at a later time. Never provide a door to door solicitor with a check or credit card number. As you may know, identity theft is the fastest growing crime internationally.

There may also be a correlation between someone soliciting and persons casing your house and neighborhood for burglaries. Even if you do not give someone money or buy their products, someone else in your neighborhood may become a victim. Call the police anytime you feel the person or solicitation is possibly illegal. It is also a good idea to let your neighbors know, in case the solicitor tries to victimize them also.