Blocking SumbleUpon

based on this
http://www.whois.ws/whois-ip/74.201.117.232/

NetRange: 74.201.0.0 - 74.201.255.255
CIDR: 74.201.0.0/16

so you need to block those set of ip addresses via .htaccess.
once you do this other sites hosted on those ip address can't access your site.

if you have existing .htaccess file better download that your computer first then add the lines:

order allow,deny
deny from 74.201
allow from all

if not existing:
create a file .htaccess

the content of the .htaccess file is this:

order allow,deny
deny from 74.201
allow from all