Teenagers have been congregating in front of my apartment building, making noise, dropping cups and cigarette butts in our garden, etc. Many of them sit on the wall between our property and the sidewalk. One of my neighbors has begun to smear machine oil, to damage the clothes of the wall-sitters and thus discourage their activity. Most of the building’s residents think this is too harsh. Is our neighbor permitted to act this way? May we demand that he stop? Answer:
[What follows are some halachic thoughts, not a formal ruling or legal advice.]
311 - Chasing after a Child during Kedusha
313 - Zimun for a Sephardi, an Ashkenazi, and a Katan
It seems that the use of the wall is more a symptom of the problem, which is the congregating in front of the house, than the problem itself. People of any age and style may congregate on the sidewalk in front of anyone’s house. On the other hand, they do not have a right to make noise that is inappropriate for the time and place in question or leave litter. One may call the police if norms are ignored; it is their job to find the balance between the interests of the various parties.
Your neighbor is performing what Chazal called "avid inish dina l’nafshei" (taking the law into his own hands), which is permitted rather than suing in beit din in certain cases (see Bava Kama 28a; Shulchan Aruch, Choshen Mishpat 4:1). While it is normally forbidden to hit someone (Devarim 25:3; see Sefer Hachinuch 595) or damage his property (Tur, CM 378), it can be permitted in the context of avid inish dina l’nafshei (Bava Kama ibid.). Is your neighbor acting within halachic limits?
A victim is allowed to take physical actions against a trespasser, even when the latter is not causing specific acute damage (Bava Kama ibid.). However, this is only when he cannot succeed in stopping the infraction without damaging the perpetrator (ibid.). Your neighbor should not just put on the grease without first warning trespassers (a clear sign is possible) to stop before having steps taken against them. Also, the rationale and consequently the limitations for this extraordinary permission are important. The Yam Shel Shlomo (Bava Kama 3:9) explains that it is predicated on the fact that the perpetrator is in the midst of violating a clear aveira and ignores the victim’s protest, so that the victim can do afrushei me’issura (preventing another from sinning). If the violators are not made aware that they are actually sinning, these steps are inappropriate (see also Imrei Bina, Dayanim 9).
Your neighbor appears to be wrong on other grounds. By putting not easily seen grease on the wall, "innocent people" can be affected. This can include passersby who get too close to the wall and become dirty. It also impacts the people of the building. It apparently stains the wall, neighbors can be dirtied themselves, or they just don’t want a reputation of being "from the mean building." In general, in matters of joint property, the rules of engagement are usually that the majority decides (sometimes there are nationwide or municipal rules or accepted practices).
Regarding another factor, we lack sufficient information. When a wall separates between private property and public domain, it is not always clear on whose property it sits. It is then possible that the wall-sitters are not trespassing. The contractor who built the building ostensibly built the wall on behalf of the homeowners. However, it is quite accepted for people who want to do so, to lean and even sit on such a low wall, and permission is assumed. There are even precedents in Halacha to make such things a right that the public can demand (see Bava Batra 12a; Bava Kama 81b) except when it is hurting the owners (e.g., the wall is getting weak). Considering that it is the assembling of noisemakers/litterers that it is the real problem, it is not clear that the building, if they agree, could legally make sitting on the wall forbidden per se.
Although we tend to disapprove of your neighbor’s actions, we cannot tell you anything conclusive without hearing his side and knowing the parties. Of course, act with wisdom and sensitivity.