Someone (neighbor) is interrupting my music? How do I prevent that?

Are you saying they are connecting to your Speaker over Bluetooth so they are controlling your speakers music?

If that is the case you’ll need to take the speaker away distance wise the prevent them from accessing it. The rub here is if you leave the speaker on and turn off or remove your system so it losses connection someone can hijack the speaker.

Some newer speakers use the newer Bluetooth standards which allows one to bond the speaker to a given device and only when you press a button on the speaker do you allow other systems access to it.

Otherwise you could try just using the audio cable connection from your system to the speaker so it’s hardwired to the speaker.