Using tribble as a work-around is hardly something that F2P newbies are going to know about.
F2P is like online retailing. Start with making a relationship with the customer, then when they are going to purchase their weekly groceries, show them the shiny new raptor for the price of a loaf of bread. Respec should be the 'weekly groceries', a reason to go into the online shop periodically, and a way to make them comfortable with the process - reducing the cost would be a sensible choice.
More than that, if the devs give a 'freebie' respec (or whatever), then the player should be instructed to use the shop to obtain it. Similarly, the player shouldn't be able to add '8 respec tokens' to their basket - let them purchase one at a time, make them a repeat customer.