Could reduce land or service value and assign it to the capital, representing capital interests completely bypassing the vassal->liege relationship. (monetarily no different than more tax...)
You could depress the market availaibility of goods locally - either actually knock everyone down a market class or do it by category, depending on what you want to happen. That'd represent merchants/traders actively not even bothering to sell locally.
More detailed approach may be adjusting "capital" demand modifiers up and local modifiers down - selling gets you not as much locally as going to the capital - but you might have to inverse that for buying things locally to have the full effect - that may or may not get weird...