We will explore a few possible ways to exempt you from returning the items.
Let us assume that your neighbors are improperly taking advantage of you. Does that justify your stopping to return their toys in order to get them to change their behavior? At first glance, this seems like nekama (revenge), refusing to do a favor that you would ordinarily do for your counterpart because of grievances against him.[1] On the other hand, several sources indicate that nekama applies only when one is punishing another for past behavior, whereas it is permitted to take unpleasant steps to try to dissuade someone from continuing his improper behavior or for another positive, not spiteful, reason.[2] Precedents for this rule include the permissibility of telling lashon hara to protect one’s legitimate rights[3] and steps that David HaMelech took against those who tried to harm him.[4]
Thus, since you have a need to stop your neighbors from abusing your goodwill and detracting from your tranquil use of your property, you could claim that you are allowed to take the measured but unusual (for you) step of not promptly returning the things thrown into your garden. However, in this realm, there is likely a distinction depending on the level of need and the nature of the steps contemplated. There is also a difference between refusing to do a favor and acting in a way that would usually violate a Torah law, e.g., refusing to do hashavat aveida.[5] Therefore, it is important to determine if the mitzva of hashavat aveida is obligatory in this case.
There is a question as to what hashavat aveida requires of the finder. Is he obligated to return the object to the owner, or must he simply enable the owner to retrieve it?[6] The stronger position, in our view – which is reportedly endorsed by Rav Moshe Feinstein and the Chazon Ish[7] – is that the finder does not have to deliver the object.[8] You imply that letting your neighbors know that they have to come pick up the toys would suffice as impetus for them to change their ways,[9] and there appears to be a halachic basis to allow you to take that step.
Even if one wants to be stringent on the above issues, we should consider whether your neighbors’ pattern of behavior falls under the category of aveida mida’at (“intentional loss”). There are different levels of aveida mida’at. One is when the owner of an object demonstrates, by allowing it to be in a precarious situation, that he does not care if the object gets lost. In that case, there is even an opinion that one is allowed to take the object for himself.[10] Your case does not fall into this category, as your neighbors want the toys back; they are not overly concerned about the fact that the toys are being thrown from their home because they rely on you. However, the Shulchan Aruch[11] writes that although we assume that an owner is generally not mafkir[12] his object, if he does not take precautions to protect its disappearance, he loses his right to require the finder to bother to return it. This seems to apply in your case. Your neighbors could argue that they try to limit the children’s throwing of toys, that you cannot blame them for lack of success, and that you are therefore required to help your counterpart, as hashavat aveida requires (even a hundred times[13]). Even so, it appears that in this case, there is no aveida at all because your neighbors always know where to find their objects, and they therefore have the responsibility to come get them. Thus, there is another reason to exempt you from doing more than returning the toys when they come for them.
In summation, there are ample reasons to allow you to tell your neighbors that they will have to come collect the toys. That being said, we urge you (who know the dynamics of the case and the relationships involved) to consider whether the situation is acute enough to justify those steps and whether your idea is the wisest way to deal with the issue.