There is no guarantee that this will stop them but it may avoid the more stupid ones.
For every page that has an email address add the lines in the header
<script>
function mailmember(name,host){
parent.location.href='mailto:'+name+'@'+host+'?subject=BCS'}
</script>
And where you want an email address add the
<a onclick="mailmember('john.smith','host.co.uk')">john</a>
This will not work if the person does not allow javascript perhaps for security reasons
If you want to put an email address on a page use @ instead of @ in your code it will be viewed as @
Information on special charters can be viewed at http://webdesign.about.com/od/charactersets/l/bl_htmlcodes.htm
You could also add format tags
eg <font color="green">John.smith</font>@<font color="blue">host.co.uk</font> would produce John.smith@host.co.uk
Fully as
<a onclick="mailmember('john.smith','host.co.uk')"><font color="green">john.smith</font>@<font color="blue">host.co.uk</font></a>
Try it john.smith@host.co.uk