Outlook 2016, 2013 and 2010 by default will allow attendees to propose new times for meetings. But by using Calendar options in Outlook, you can also prevent any attendees from proposing new times for the meetings. This can be done by navigating to Calendar and then right clicking and selecting Calendar Options from the context menu.
In the Outlook options, navigate to Calendar Options and un mark the check box with label as Allow attendees to propose new times for meetings.
After making this change, click On button available at the bottom of Outlook Options screen to confirm and save the changes. This would never allow any attendees to propose a new times for a meeting request in Outlook.