Bulk Deleting All Messages in a Folder in Microsoft Outlook Web Access
UPDATE
Just right click on the desired folder and select Empty Folder
Like dean said, you can hold Ctrl to select single items or even Shift to select multiple items.
I haven't used OWA in a long time but a little trick I used to do in Firefox was to use this javascript bit to select all of the checkboxes
javascript:var f=document.activeElement.childNodes1.contentDocument;var c=f.all['MsgID'];var i;for(i=0;i
type it in the address bar and hit enter, it should select all of the checkboxes.