1 -- Reduce direct prices. At some price, people will buy..
2 -- Use resale restrictions + ROFR to make re-sale unappealing.
Combine 1 and 2 as necessary until you have the balance of direct sales you want.
Of course, most people purchase while visiting Disney, without ever researching the re-sale market as an option. And that ignorance is Disney's ally as well.
But now fast forward to the late 2030's, early 2040's... Direct purchase gets you access to 20+ resorts. Re-sale of a legacy resort locks you out of the newest resorts, on short contracts, and covering only about 9 out of the 20 resorts.. re-sale of the non-legacy are 1-resort-only contracts. So at that point, buying direct becomes a massively different product than buying re-sale.