Note: this will stop Outlook and Exchange from creating tentative appointments on the calendar automatically, however, the tentative appointment will be created at the time you select the meeting request in your Inbox.
AddNewRequestsTentatively only works with resource mailboxes in newer versions / builds of Exchange server, including Office 365's Exchange Online.
Exchange Server 2013
The administrator can use this cmdlet to prevent tentative appointments meeting requests received from users outside of your network.
Set-CalendarProcessing -Identity "your name" -AddNewRequestsTentatively:$false
This cmdlet will block external meeting requests.
Set-CalendarProcessing -Identity "your name" -ProcessExternalMeetingMessages $false
Change Outlook options for Tentative Appointments
You can configure two Outlook options to reduce the annoyance level of meeting spam.
In Options > Calendar, turn off default reminders. While this will affect appointments you create, you can use an ItemAdd macro to set reminders on appointments marked busy.
If using POP3, IMAP, or Outlook Connector accounts, you can turn off the 'sniffer'. With the 'sniffer' enabled, the meeting request is automatically added to the calendar as tentative a few minutes after arrival. When it is disabled, a tentative appointment is added to the calendar when you select or open the meeting request.
Turn off the sniffer at File > Options > Mail > Tracking section (Outlook 2010/2013) or Tools > Options > Email options > Tracking, do not select the option to process requests and responses on arrival.
Depending on the version of Exchange server, users may be able to also control this setting in OWA. In Options, Setting, Calendar, under Automatic Processing, disable the option to Automatically place new meeting requests on the calendar as Tentative appointments. This is not available in Outlook on the web (Office 365).