Personally, I like Derstrom's answer for its simplicity about using Altium's built-in solution. However, if you need keepout clearances not on your board edge, it won't help you there.
What I do is create a rule for my clearance on my outer layers, called say BoardOuterLayer_Clearance:
Then I create a similar rule called BoardInnerLayer_Clearance, make it lower in priority, to catch the other layers, and this second rule is exactly the same as the first except second object matches "All."
This lets me set the pullback for inner layers to something higher, say, 15mils, since often the board houses recommend this.
Then I just need draw a boundary around my board and any pours or online routing rules will respect this. Here you can actually see I put my KOR already pulled back at the inset, so a bit of overkill here on this board. But in this image, the polygon pour goes all the way to the board edge, so the pour is respecting the rule:
Of course, the real beauty of this method is that it will work everywhere. So here I have a footprint for a screw hole, with silkscreen to indicate to the designer but there is also a circle drawn in keepout layer there that will keep any screw heads from eating through and biting into copper.
The rule handles this for me and all I have to do is draw keepout wherever I need it once I've set up the rule.