You can control this via Settings. If using Classic Outlook, go to File > Options > Calendar > scroll down to the Automatic accept or decline section and click the Auto Accept/Decline button. Therein, select the first two checkboxes:
If using the New Outlook or OWA, go to Settings > Calendar > Events and invitations > scroll down to the Invitations from other people section and check the Automatically decline invitations that conflict with existing events.