So this doesn't work?
Get-MailboxExportRequest | Remove-MailboxExportRequest
If not, then use adsiedit.msc
Change to the well-known naming context "Configuration"
Then open the tree > Services> Microsoft Exchange > Your Org Name > Mailbox Replication > Mailbox Export Requests
Delete any entry under there. DO NOT delete the "Mailbox Export Requests" folder, just any requests listed under that folder. Be careful! Delete only the export requests, not any parent container.
When you get to the Mailbox Replication Container, you will see a list of sub containers, then expand "Mailbox Export Requests"
If you don't see then post the exact error messages you are seeing.