[natch]

The police probably won't act on it since they perceive phone theft as small stuff. It might be up to you to get them to act. What country is this in?

You might try to get more information on who you are dealing with. This is tricky to do without tipping your hand that you are doing it. But if you can find the right person such as a landlord, or a parent of the culprits, neighbors, etc. who could exert social pressure (or not) they might (or might not!) be helpful. Really depends on the neighborhood and what kind of community it is, and who they are. Be very wary. Just ideas... take them with a grain of salt.