You can't really stop the spoofing party from sending out emails under your accounts since anyone can do this. There are steps you/your client can take to validate the authorship of the emails if you are prepared to do so.
Some methods require you to take some steps to verify the origin of emails sent on your domain, while you can also work with your registrar/ISP to look at practices in place, but this would depend on the service you are getting.
Some practices you can implement include Sender ID or domain signing. Obviously this would require your clients email server to look for these specific signatures, which you may or may not be able to implement. A simpler solution would be for your clients to look at their spam filter policies and possibly inform them of emails which are known spam and get them to filter these out. All this information should be available in the email header making it easy to block out.