A historical conflict that got resolved.
We've seen this play out in our own history.
- US vs. England during the revolution
- US vs. Russia in the Cold War
- Britain colonizing India but then slowly losing control because of peaceful resistance
There's a wealth of real world historical events you could pattern the conflict after, that gives you more or less the desired relationship between one society and another.
By having a conflict like this, you show that the species is capable of matching humans. This definitively proves that humans can't simply walk in and take over; theysome of them already tried and failed. The fact it's a historical event easily explains why they weren't completely outmatched by technological superiority, and it establishes a believable groundwork for any kind of treaty or diplomacy. Whatever agreement was established at the end has just held on because no force has decided to challenge it. (Maybe for some reason it simply isn't in anyone's interest to do so yet.) After many years in this situation, the creatures are basically an established nation. This is completely normal in our everyday world. Maybe around a century or two ago would be perfectly reasonable.
This also makes for good story telling and themes. It plays nicely into a "nature vs. technology" theme, if you make it a military conflict where they nearly won or fought to a standstill. (Are they still a match now that humans have advanced?) It gives you a great set up for classic drama between characters in both groups; there's a natural reason for enmity between them while still allowing for good relations as well.
This wouldn't necessarily prevent clandestine attempts to capture and study the race, but it would be enough to prevent it on any kind of grand scale. Those clandestine attempts at research could be a driver of your story's central conflict, if you choose, or you could simply ignore them as unknown to your characters and irrelevant to your story. There would also be attempts to study it using humane means with the consent of participating creatures; the creatures can just refuse wholesale then.
The bottom line, I think, is that it simply isn't believable that humans don't want to study the creatures or their ability. Instead, give humans a reason to think that just isn't going to work on a large scale. Come up with a reason that it makes more sense for humans to make peace with them, or at least it did at one time.