That's definitely one I have struggled with in the past. On my site (skunk.guru) I simply force them to send the message via the contact form and don't list my email address for them. The only way they get access is to send a contact form and get my email from the reply.
However, there are now a large number of tricks to display an email address on the web without it being as susceptible to spam bots/crawlers. Unfortunately, I'm not sure how well it will mitigate existing spam to change it, since the address has already been crawled, but it may help prevent more bots from crawling it and help with future clients to implement one of these measures. Note that nothing will stop a human from reading an email address and manually adding it to their spam list, so no matter what, if you post an email on the web, you leave it open to the possibility of spam.
Here's a good site with some of those tips/tricks on how to avoid your email getting crawled.
How to Hide your Email Address from Spam Bots