It looks to me as most effective solution what AlexB77 said:
RewriteCond %{HTTP_USER_AGENT} Firefox/27\.0 [NC]
RewriteRule .* - [F,L]
And it works if you try to visit your website with User Agent Firefox 27.0
Now it is to see of it will works for blocking Adsense Attacking Bots.
If someone is still looking for xlhost IP ranges i have collected some of them using As number AS10297. It is nummber from XLhost:
<Files ~ "^.*$">
order allow,deny
allow from all
#XLHOST IP RANGES
deny from 209.51.197.0/24
deny from 209.190.121.32/27
deny from 209.190.0.0/17
deny from 173.45.64.0/18
deny from 64.79.64.0/19
deny from 64.79.89.0/19
deny from 64.79.85.0/19
deny from 207.182.128.0/19
deny from 173.244.160.0/19
deny from 206.222.0.0/19
deny from 207.182.128.0/19
deny from 209.190.116.0/24
deny from 209.190.70.0/24
deny from 209.51.192.0/19
</Files>
[edited by: dolcevita at 1:03 pm (utc) on Apr 20, 2015]