I did the same thing to my Grizzly Spindle Sander last year. I bought a 2 1/8” bearing to put under the drum. I removed the belt that drives the oscillating portion of the sander and it leaves a smoother finish with no snipe. I also bought 2”x2” 36 grit sleeves and run two on the drum. The bottom sleeve does all the work and the top sleeve is just there so the drum squeezes evenly.

I cut my drum down on my bandsaw to the height I needed so I could use the big washer and a wing nut.

I de-oscillated my Ryobi oscillating drum sander, it was fairly simple to do. It was last summer when I did this but I will try to explain the procedure from memory. From underneath I removed the motor and mechanical mechanism so I could see what made it oscillate. I found out that it was a rotating disc with a cam lobe on it that caused the  drum shaft to rise and its weight was what caused it to fall.  I turned the cam to lower the drum to the lowest position. Then I could  pull the drum and shaft to the uppermost position which exposed approximately an inch of the shaft.  I then cut a piece of 1/2 inch ID PVC pipe to the length of the exposed shaft.  Then I cut the side of the PVC so that when you looked at the end of the PVC it would look like the letter C.  the opening was just enough to allow the PVC to snap on the shaft and stay there. The drum is now raised to its highest position and can't fall when the cam rotates to the position for it to. I Now have zero oscillation.   I know that all oscillating sanders aren't made the same but if you have one like mine this may help.

Best if the bearing thing do not turn with the drum, but is stationary or at least free turning.

This is what I bought to use as a rub collar. I pressed the smaller bearing into the larger one so it would fit the 1/2” shaft on my sander. I stacked two of these together to get the collar up above the sanding table. I put a thick 1/2” washer on top before putting my sanding drum on so the bearing does not spin with the shaft. Very simple to do and works great.

Autumnarcher, you may want to check this out.  It will work with or without oscillation if you make the rub collar tall enough.
http://www.tradgang.com/tgsmf/index.php?topic=167155.msg2858922#msg2858922

Yup thats the video I saw that gave me the idea to modify mine. Worked on it over the last few days, as well as making a new template to go with it. 5/16" aluminum plate for the base. The sander I used 2 flanged bearings with a nylon spacer so the bearing does not spin with the sander shaft. Works quite well, although it took a lot of trial and error to get the configuration right. My biggest challenge was making the bearing itself, out of 3/4" plywood.
Cut out ab disc with a 2-1/8" hole  saw. Problem being getting the center hole exactly centered. the pilot bit in the hole saw is 1/4". I used a Farstner bit to cut out the hole for the flanged bearings, and was rough centering it. I drilled pilot hole out to 1/2", cut a 1/2" plug and tapped in then marked center for the tip of Forstner. Rather painstaking, but eventually got it. Fun part was got to play with several tools I havent used in a while.
Heres a couple pics of the finished setup. I run a 36 grit sleeve on the sander. Thanks to all for the input. Was def helpful.