What exactly do you mean by joining? That your gateway is receiving 433 RF signals from your neighbours which you then see as MQTT messages, or joining in another way?
If you are also receiving RF messages from other devices than your own, that is completely normal, as the RF receiver is a passive receiver which picks up ANY 433 RF signals in the vicinity. Pretty much the same with the IR or BLE gateways.
With the latter, the BLE gateway, it is possible to set a white-list or a black-list, which then filters out the received devices to only create MQTT messages for the devices you want, my defining the Bluetooth MAC addresses.
For the RF gateway though there are no white-list/black-list options (yet) - anyone please correct me if I’m wrong here and there are already filtering options for protocols and such - as the unique identification of received RF signals is not as easily possible as with BT MAC addresses. The only thing I have found here so far is the leading digits of the value to correspond with some of my different RF devices.
So the only possibility for you might be to implement a filtering option on the receiving end of your RF gateway MQTT messages, to only consider messages relevant to your setup.
I hope I understood you correctly and the above is helpful.