Is it legal for the property company to allow others to spray advertisements on the external walls of residential buildings without authorization?

This is illegal. Article 55 of the Regulations on Property Management stipulates that those who use the parts and facilities of the property to conduct business shall go through the relevant procedures in accordance with the regulations after obtaining the consent of the relevant owners, owners' congress and property service enterprises. Owners' income should be mainly used to supplement special maintenance funds, and can also be used according to the decision of the owners' congress.

Article 66 of the Property Management Regulations stipulates: (3) Unauthorized use of parts and facilities of the property. Individuals who commit one of the acts listed in the preceding paragraph shall be fined 1000 yuan or more and 10000 yuan or less; If a unit commits one of the acts listed in the preceding paragraph, it shall be fined between 50,000 yuan and 200,000 yuan.