If you got an email similar to:

[Acorn]

Hahahaha.

Members don't get "stolen." People would rather stay where they are on a message board even if they are semi-miserable than migrate to a new one. So if you are getting people coming from a particular other board that must mean things are REALLY s****y over there. With an admin like that I'd believe it.

 

[VirusZero]

Somewhat like Acorn said, members don't get stolen. It's the members themselves that decide where to go and who to join.

So if I got an email like that I probably wouldn't bother to respond. But in the odd chance I did, I'd probably say something like:
"People are being stolen and dropped onto my board? Say what now? Surely this must be a mistake. I had thought all these people had joined willingly."