spam with my own domain

There's no fix for this. Anyone can forge the From: address in an email, and only a technically sophisticated user can detect the forgery.

You could use DKIM and/or SPF to increase the chance that ISPs will detect the forgeries and block the emails, but this will only work sometimes.

It could be that what you're seeing is a spammer using your domain name as the "From" address for a particular set of mails. This is done as a method of bypassing some spam filters (although I don't think it's particularly effective these days, as commercial mail filters still seem to be quite good at recognizing the mails as SPAM)

There's no way to directly stop the spammer from doing this, however I wouldn't worry too much about your customers seeing this as a negative, as it's a common phenomenon (a domain I've got gets this on a fairly regular basis) and not within your control to prevent.