if you go to "estate" in the top bar and click "manage" then click "tabs" and click the little pencil after the tab that you want to freeze there should be a slide bar that says "freeze" if you move it over i think it will freeze that tab.      

You can freeze your entire account by going to Account > Settings. To freeze individual horses, you need to freeze specific tabs. This can be done by Estate > Manage > Tabs. You'll have to move the horses you want frozen to the tab(s) you freeze.