Hey there LeoniE24,
I apologize about the late response, thank you for your patience! Were you able to remove your neighbor's Chromecast?? If not, I'd be happy to help out, as I can imagine you don't want them on your Wi-Fi. Let me just ask you some questions about what's happening:
I look forward to your response, and if you have any other questions or concerns for me, please let me know!