When someone sends you a meeting request, you have three main options: accept, tentative or decline. Additionally (and if this option was not unchecked by the sender), you can propose a new time.
Accepting the invite means it will appear in your calendar. Selecting the option ‘tentative’ means it will also appear, but it will be marked clearly as tentative. And finally declining removes the event from your calendar all together.
However, I was in the situation where I wanted to decline a meeting, but still keep this calendar item on my agenda. It was a meeting/event I was not attending to, but I wanted to be remembered this item takes place.
The steps to take to decline a meeting but still keep the meeting in your agenda are the following:
1) Since you want to decline the meeting, you first click on Decline and ‘send the response now’ so the meeting organiser knows you can’t attend
2) Go to your deleted items and find the meeting you just declined. Open it and click on ‘accept’ but choose ‘do not send a response’. Now the item is back in your agenda.
3) Optionally, you can choose to ‘show as free’ so others see you are available at that time
End result: the meeting organiser receives a notification you are not attending; but the meeting is still in your agenda. This should work fine in all version of Microsoft Outlook and even on the web version (i.e. Office 365).