Just make the owner cannot sell/transfer ownership for 1 year and make him pay 12 months monthly fee from the start. If after 6 months if s/he wants to sell/transfer ownership then he'll have to pay 20x the price of land. After 9 months = 10x, and only after 12 months s/he can sell without paying.

The new buyer would have to do the same as well for 12 months.

This will not kill house flippers outright, but it'll discourage average joe house flippers (95% of them) since it requires a major commitment.