Yes, it is a choice. Or more specifically, you must pull the creature, but you can pull him 0 feet.
Thorn Whip
You create a long, vine-like whip covered in
thorns that lashes out at your command toward a creature in range.
Make a melee spell attack against the target. If the attack hits,
the creature takes 1d6 piercing damage, and if the creature is Large
or smaller, you pull the creature up to 10 feet closer to you.
The spell does exactly what it says. Many spells and abilities give you the ability to choose, by including the appropriate wording. This one specifically allows you to choose the length you pull the creature (0-10 feet).
This will not work; Readying an Action to Cast a spell requires the use of Concentration
Ready
[...]
When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration (explained in chapter 10). If your concentration is broken, the spell dissipates without taking effect. For example, if you are concentrating on the web spell and ready magic missile, your web spell ends, and if you take damage before you release magic missile with your reaction, your concentration might be broken.
—Actions in Combat, Player's Handbook, pg. 193
Unfortunately, Spirit Guardians is a spell that requires the user to maintain concentration. So if you tried to use this sequence of events, Spirit Guardians would end the moment you tried to Ready the Thorn Whip spell.
"Okay, but what if Thorn Whip was a Reaction spell that wouldn't interrupt Concentration?"
For the sake of argument, let's assume that Thorn Whip instead had a Casting Time of "Reaction, used when a target runs away from you". This would mean that we would no longer have to use the Ready action to prepare it, which means no longer needing to maintain concentration on Thorn Whip, meaning no longer breaking concentration on Spirit Guardians.
In this case, you would be able to trigger the second source of damage: this is the first time this turn that the creature has entered the Spirit Guardians' radius; previously, the creature started its turn within the radius, but it did not enter during its turn. So using Thorn Whip in this manner would deal the second source of damage that turn.
You're overcomplicating this
You don't need to bother with trying to ready an action. The effect of Spirit Guardians, in addition to dealing damage, is to slow movement of creatures affected by half. Most hostile creatures have a movement speed of 30-35 feet per round, which means that at half speed, a creature running directly out of the Spirit Guardians probably doesn't get more than 5-10 feet away from the edge of the AOE. As a result, using Thorn Whip on your own turn, as a regular Action, instead of trying to ready+Reaction it, is perfectly usable to pull them back inside the Spirit Guardians, triggering the extra damage. And if they're more than 25 feet away from you (meaning Thorn Whip cannot pull them inside the AOE), you can just move a few feet closer before casting it. This will cause them to take damage from being moved within the radius, and then take damage again when their turn starts.
So overall, this method deals the same amount of damage as the method you're trying to employ, but it doesn't require you to use your Reaction, it doesn't require you to invent a fake spell that is "Thorn Whip, but cast as a Reaction", and it is easier to make sense of in the rules of the game.
Best Answer
Nope.
The pull effect of Thorn Whip is contingent on a successful melee spell attack against a creature who is Large-sized or smaller.
If both conditions are true (the attack hits and the creature is no larger than Large-sized), then the pull effect would occur whether or not the creature takes damage.
Note that the spell doesn't explain how or why the creature is pulled. Do the thorns grip them? Does the whip wrap around them? Is it some unseen magical force? Regardless, spells do what they say they do. A hypothetical DM could rule otherwise, but by RAW, the creature is pulled simply because the spell says so.