I am using Outlook Webaccess. unfortunately the calendar shows up around 5MB of data. Due to this I am not able to send mails since there is a restriction on my user account.
Is there any way of deleting all the calendar items at once?
I can't fine a way to delete several items at once from the calendar using OWA.
I would recommend you look elsewhere to delete data. Chances are that 5MB isn't use much space relative to your total allowed mailbox size. Check your Deleted Items and Sent Items folder for mail you no longer need. Also look for mail with attachments you don't need.
I found that you can delete multiple items from Outlook Web-access by opening your calendar, clicking on the Search button in the toolbar at the top and then searching for just calendar items (to find all on my Calendar I put my name in the "Send To" field). From the search results you can select multiple items and delete them by right clicking and selecting "Delete"
Note: This appears to only work in IE