The only way that I can see to do it is to fill the relevant time periods with nominal 'meetings' (such as 'Lunch', 'Playing Golf' or 'Watching Teletubbies') and then enable Auto-Decline.
http://www.uwec.edu/help/email/automaticmeetingacceptdecline.htm
Outlook will then see that any new meeting request conflicts with an existing 'meeting' and thus decline the request.
(PS: When posting questions about Outlook (or any other Microsoft Office program) it helps to know which
version you're using. Instructions for one version can often be completely meaningless in a different one!)