wilderness - 10:26 am on Jan 4, 2013 (gmt 0)

# deny blank UA; IP ONLY
RewriteCond %{HTTP_USER_AGENT} ^-?$
RewriteCond %{REMOTE_ADDR} ^70\.37\.([0-9]|[1-9][0-9]|1[0-8][0-9]|19[01])\.
RewriteRule .* - [F]

FWIW, it's my understanding if your server has installed the latest version of Apache (2.4-?) that you may use CIDR.

My el-cheapo still has 2.2 installed.