This isn’t a perfect solution, but I hope it can help someone else - the idea is to head off some spambots before they hit your site or even get as far as Bad Behavior (which is quite superb, BTW).
It looks like this:
RewriteCond %{REMOTE_ADDR} ^64.140.49.6([6-9])$ [NC,OR]
RewriteCond %{REMOTE_ADDR} ^85.114.141.114$ [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^NPBot [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^WebEMailExtractor [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^EmailSiphon [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^NICErsPRO [NC,OR]
#### --- etc. ---
#### and then pokes them in the eye with a forbidden header.
RewriteCond %{HTTP_USER_AGENT} ^[a-z]+$ [NC]
RewriteRule ^.* - [F,L]See attached for the full code. As ever, your mileage may vary… (mine certainly does).