It's handled in the java file NGCAddStandardStartingScript.class, which is in com.fs.starfarer.api.impl.campaign.rulecmd (this is in the starfarer.src.zip found in the starsector-core folder). You'll need to compile a new java file with the line
if (spacer) {
new SpacerObligation();
}
removed. Best to create a mini-mod for that I think.
Another option would be editing the rules.csv, look for $ngcSpacerSelected = true. Note that setting that to false affects other things too besides the debt.