I'm starting to think that ultimately, the reasons for coming to the Americas are going to be so numerous and the people of Europe so diverse that trying too hard to restrict major, serious expeditions is a fool's errand.
No matter what warnings we make, no matter how much we obfuscate what can be gained, no matter how scary we make things sound, someone is going to be stupid, greedy, adventurous, or desperate enough to try for another endeavor anyway, and what might ward off a lot of people will just attract others who have different motivations.
More efficient I think, is going to be working to rapidly strengthen Union's position and destroy or weaken its existing enemies, as well as sending out scouts further to look for more allies. All of that will improve our ability to project power and wrangle any new colonies that end up being set up.
At the very least, the knowledge of the New World having a bunch of Monsters and the fate of Johnstown should mean we have some time before serious colonial expeditions get sent out. Assembling the people and resources needed for a successful colony is going to take some time and a great deal of money after all. People can just examine how much was set up for Johnstown, and look at what happened to them.