Possible login/spammer hack. Looking for preventative solutions. vBulletin 4.2.1 site

[JohnD5000]

Over the last 2 weeks I have received 12 vBulletin database errors of the following type:

Database error in vBulletin 4.2.1:

Invalid SQL:
SELECT userid, usergroupid, membergroupids, infractiongroupids, username, password, salt FROM user WHERE username = 'basket compens??es isabel marant';

MySQL Error : Illegal mix of collations (latin1_swedish_ci,IMPLICIT) and (utf8_general_ci,COERCIBLE) for operation '='
Error Number : 1267
Request Date : Tuesday, November 26th 2013 @ 02:16:49 AM
Error Date : Tuesday, November 26th 2013 @ 02:16:49 AM
Script : http://www.empirisoft.com/support/login.php?do=login
Referrer : http://www.empirisoft.com/support/me...75-AryanDuncan
IP Address : 142.0.143.20
Username : Unregistered
Classname : vB_Database
MySQL Version :

Please note the funny text that someone is trying to use as a username. Also, all 12 errors originated from the same IP address.
I think this is an attempt by a spammer to hack a username on our site. Is there any solution/add-on to prevent this type of attack in the future?

​Thanks in advance for any and all suggestions.