http://exchangeserverpro.com/list-users-access-exchange-mailboxes/

This command will export to CSV a list of any mailboxes where other users have permissions to access them, and will also list what level of access those users have.

Get-Mailbox | Get-MailboxPermission | where {$_.user.tostring() -ne "NT AUTHORITY\SELF" -and $_.IsInherited -eq $false} | Select Identity,User,@{Name='Access Rights';Expression={[string]::join(', ', $_.AccessRights)}} | Export-Csv -NoTypeInformation mailboxpermissions.csv

remove FullAccess accessrights

Remove-MailboxPermission -identity Mailbox -user Username -AccessRights FullAccess -InheritanceType All -confirm:$false

remove SendAs accessrights

Remove-MailboxPermission -identity Mailbox -user Username -AccessRights SendAs -confirm:$false

Removing user from Manage Full Access Permission in all mailboxes

Get-mailbox -resultsize unlimited | Add-MailboxPermission -user 'Domain\user' -Deny -AccessRight 'FullAccess' -confirm:$False