Yes, of course it's possible, with airplane configuration editor.
In real life, 737s don't use ULD containers at all, and all the 737s supported by GSX are configured to not use one, and use only the belt loader.
HOWEVER, I found that, of ALL the supported 737s (default, iFly, Wilco, and PMDG), ONLY the 737-800 variant of the PMDG (other variants are fine), didn't explicitly exclude ULDs, so the selection was back to "Auto", which means "Depending on door's height".
So, even if you can configure it yourself with the editor, maybe it would be best if we had all 737 to work the same, and exclude ULDs on the PMDG -800 variant too.
Will be featured in the next Live Update, of course.