If you are using SAS 9.1.3 or SAS 9.2, then (per this Tech Support note) you should be able to use the System Options to alter the RTF margins starting in SAS 9.1:
options topmargin=.5in bottommargin=.5in;
If you are using SAS 8.2 or want to use a custom style template for changing margins, then this Tech Support note outlines the methods that you would use via style template: http://support.sas.com/kb/24/033.html
Do remember that ODS RTF creates RTF control strings that are designed to work best with Microsoft Word. So if you are creating RTF output and using some other RTF viewer (OpenOffice, ClarisWorks, etc), then you may have to work with Tech Support if the RTF control strings for margins are not respected by those viewers.