Am I sinful for not paying people who clean my car without my permission in a public street?

"What is known by custom is as if it were stipulated by condition." If it is customary in the questioner's country to wash cars for a known fee unless the owner explicitly states otherwise, then the owner's failure to inform them implies permission for them to perform the work and their entitlement to the fee. Your refusal to pay without having informed them is considered an injustice and an usurpation of rights. You must repent and return the rights to their owners, or donate the amount on their behalf if returning the right is not possible. However, if this is not the prevailing custom, or if you informed them of your unwillingness and they ignored your request, then you are not obliged to pay. To avoid payment, you must inform them of your unwillingness.