Hi Anwar,
Yes, you can delete them in bulk on CRM 2011 online. There are many ways you can bulk delete records but easiest way is to use Advanced Find, put a criteria to select records candidates for deletion and then bulk delete them in one click.
Here are couple of URLs that would guide you how to do that with screen shots.
https://community.dynamics.com/crm/b/crmcustomereffective/archive/2012/06/05/using-the-quot-bulk-delete-quot-feature-in-crm-2011.aspx
http://blogs.msdn.com/b/crm/archive/2008/04/24/bulk-record-deletion-in-microsoft-crm-online.aspx
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Thanks,
Imran Abbasi