Office365 and calendar permissions

Run PowerShell and run following commands:
$UserCredential = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
Import-PSSession $Session

Remove-MailboxFolderPermission -Identity user@mycompany:\calendar -user myuser@mycompany.com

After that, re-grant the calendar permission Review, Editor or Owner to the user. In this case:
Add-MailboxFolderPermission –Identity A@contoso.com:\Calendar -AccessRight Editor,Owner -User B@contoso.com