How do I print Keynote slides full-page

First of all you go to File > Print. Right to the name of your printer there´s a blue box icon with an upside-down triangle in it. Click on the triangle. Then you should have many more options. Where it stands "Add borders around slides" the box left to it is selected. You simply deselect that box and you should get a border free printing!

i am not sure if i understand your question correctly, but are you referring to the empty spaces on the margins that perhaps you want to fill up by making the slide take up the entire space? if so, here's a solution i worked out recently for Keynote 08

1. go to File>Page Setup
2. in the paper size menu, choose custom size
3. create a new Paper Size with a name like "Handout Size" or something
4. in the printer margins, choose user defined and set all the margins to zero, or any other appropriate values for you ...

This should make the slides fill up the empty spaces. Hope it helps.