This is currently not possible with the standard iOS calendar sync.
Signing in to Facebook on your iOS device and showing the events on your calendar will synchronise the entire calendar from the server on your device, including events to which you have not accepted. Calendar on iOS or OS X does not respect the response which you have given for the invitation as a method of deciding whether to show the event on your calendar.
This is detailed on Facebook's help center with a possible workaround:
A better solution is not to use the FB calendar integration but instead subscribe yourself to your facebook events. You can do this by going to your events page on the full facebook site on a PC. Click on the cog in the top right hand corner and select export and then select upcoming events - copy the link from that and email it to yourself. Open the email on your iphone and click on the link. That will prompt you to subscribe to you FB events calendar which you should do. All your facebook events excluding declined events will now show in your iphone calendar.
The cog mentioned in Facebook's answer has been replaced by a link in the bottom right corner, "Upcoming Events".
You can provide feedback to Apple here: