How did Louis XIV keep the French nobility harmless?

By making them compete for meaningless honours in his own house. He moved the court to Versailles and required the great families to attend, where the day was structured around ceremonies - the levée, the coucher - in which the right to hand the king his shirt, or hold a candle, was a serious prize allocated by rank and favour.

It worked on two levels. Attendance was ruinously expensive, requiring a household, clothes and constant display, so nobles ran into debt and became dependent on royal pensions - and being at Versailles meant not being on your estates building a regional power base. The Fronde, a noble revolt during his childhood that had driven the court out of Paris, was the thing he was preventing, and he never forgot it.

The system cost him the intelligence a monarch needs. A court organised around flattery told the king what he wanted to hear, and he spent his last decades in expensive wars on poor information. It also concentrated the nobility in one place while divorcing them from any function, which is a fair description of what the revolutionaries found a century later - an aristocracy with enormous privileges and no remaining job.