First the much important steps:
- change the password of your webspace admintool and your FTP account also
- change the password of your WB-admin account
- check the rights of all other WB-users and restrict their rights to a absolute minimum
- clean up your installation / webspace
- if your site still becomes defaced anyway, contact your hosting provider
these are the first steps only, if it does not help, response please.
Important: use secure passwords only! At least 8 chars (mixed upper- lowercase, numbers, special chars). Never use words which can be found in any dictionary or some names, birthday, and so on..)