Get-MailboxExportRequest to see all the requests
Get-MailboxExportRequest -Status Completed | Remove-MailboxExportRequest to remove the completed.
Get-MailboxExportRequest | Remove-MailboxExportRequest to remove all of them http://www.falconitservices.com/support/KB/Lists/Posts/Post.aspx?ID=115
$mailboxes = get-mailbox
foreach ($mailbox in $mailboxes) {
new-mailboxexportrequest -mailbox $mailbox -FilePath \\server\share\$mailbox.pst }