How is it assault for the owner of the property to install a contained sound system in a building that they legally own?
All that would need to do is provide the squatters some written notice informing them that this building was selected for testing a non harmful experimental rodent deterrence prototype the purpose of which is to minimize the presence of known plague vectors. (High/low frequency devices are often used for this and similar purposes). By notifying the “residents” in advance, it establishes that they are made aware that the owner of the property, although not actively seeking the removal of their presence from the premises at this current time has given them fair warning that these devices will be installed and made operational at the provided date and they are not actually being asked to leave. The legitimate justification behind the use of the property for this purpose is to afford maintenance and safety upkeep on said property by minimizing financial losses incurred while income from the property through other means is not possible due to it being forcibly prevented from generating income due to the presence of the squatters.
Perhaps the rebuttal to this approach are zoning laws favoring the sole use of the property as purely no residential.
Another unrelated risk that the squatters open them self up to is for the owner to contact multiple strict by reputation inspection agency that is likely to find some reason why the area should be not approved for renting to individuals due to safety reasons while those flaws are amended. During this and before the owner could establish communication with several repair companies seeking quotes on the repairs needed to meet the livable standards again but document that the repairs are not currently possible due to the potential financial legal liability of performing construction while the building is physically occupied. If a building inspection agency classifies the building as unsafe for residents then anyone choosing to stay there with children would be technically liable for child endangerment and would be at risk of being reported to the French equivalent of the child protective services.
If you stare at the playing field long enough there are always legal checkmate moves in every game no matter how inconsistent or ridiculous, even the French legal system. There appears to be a pattern of justice miscarriage in the French legal system possibly calling into question whether or not any other nation should ever honor any French extradition request as long as they hold laws as silly as this and preventing someone from seeking to prove with modern forensic science whether or not they are innocent of the accusation of genetic paternal responsibility.
The flowers in their “bouquet of evidence” are less than fragrant.