Most Email programs have a spam filter, which can be adjusted from its normal default. Problem is that to increase its intensity, creates a possibility that you may have mail going into the spam box, which should have been first read. Obviously, this means that you have the extra task of checking in there.
Possibly this does not answer your question as to how to stop it in the first instance.
This is a function with your Email provider. In my case, my Email/internet provider also has a spam filter. Unfortunately, the same problem exists. I have from time to time looked in there and discovered mail which I would have liked to have read.
WLM had an interesting option, whereby you could bounce back the unwanted Email to the sender. I did use this and it appeared to work.Point there was that the sender got overwhelmed with returned Email. Once again, there was a grey area here. Many of the spam mails you receive, are anonymous, so the bounce feature cannot deal with it.
Only other option is to try and return an answer advising the unsolicited senders that they are not required.
Personally I have given up and just laboriously delete such mail now.,