I'm not familiar with a PLEXUS USB reciever, Andy, but one idea might be to change the Wireless Channel that your router receives on, so it's different to your neighbour's.
The above should hopefully solve things, but you might want to get your neighbour to secure his network, as that would also hopefully stop any interference with your own, and would be good for his own protection.
By the way, not a telling off, just an advisory, but did you know it's illegal to access another person's wireless network without their consent, as he could be facing a bill for your internet use, if he's on a capped service. Of course, if you have an agreement with your neighbour, it's sort of OK, but may still be against the T&Cs of his ISP.