This is related to Can a store sell merchandise I've left in the store? The phone in question has been mislaid and anyone who finds it has a duty to deliver it to the owner of the bench for safekeeping pending the true owner's return: if the owner does not return within a reasonable time the phone becomes the property of the bench owner (e.g. the city that owns the park).
However, the specific question here is: Where the owner has returned within a reasonable time but the possessor of the phone is now clearly attempting to steal it. Most jurisdictions recognise that a person is entitled to use reasonable force to defend their life or property. For example, the law in Australia1, is generally case law for which the authority is the High Court's decision in Zecevic v DPP (1987) 162 CLR 645:
The question to be asked in the end is quite simple. It is whether the accused believed upon reasonable grounds that it was necessary in self-defence to do what he did. If he had that belief and there were reasonable grounds for it, or if the jury is left in reasonable doubt about the matter, then he is entitled to an acquittal. Stated in this form, the question is one of general application and is not limited to cases of homicide.
So, you are entitled to do "what you believe upon reasonable grounds that it was necessary to do" to defend your property. This would include using physical force to stop their flight and return your property to your possession: it would not include force that posed real and foreseeable risk of inflicting death or grievous bodily harm upon them.
In addition, because you have reasonable grounds to believe that they have committed a crime, you are allowed to arrest them and deliver them to lawful custody (i.e. a police officer). Naturally, if you do not have reasonable grounds them you have just kidnapped them.
The consequences if you do injure them is that you can be charged with a crime (battery, grievous bodily harm, manslaughter, murder etc.) and/or be sued for damages (medical bills, lost wages etc.) in both cases you could use self-defence as a defence.
The difference between self-defence and vigilante justice is one is legal and the other isn't