Well, there are a couple of things you can do. First of all, keep in mind that a strict reading of the power suggests that the enemy must be within 5 squares before moving closer, but I admit that there is some room for interpretation. Second, keep in mind that this is an immediate reaction and therefore the ranger only gets to make one of these attacks in each full round. Third, keep in mind that it is a daily power. So, it should only effect one combat in four on average.
If you still find it problematic, there are some things you can do. For one thing, why should the enemy advance instead of taking cover? Can the enemies outflank to cut off the retreat? If the party is staying tightly packed, why don't the enemies use area attacks or forced movement to break them up? After all, the enemies should respond intelligently to the situation. Even a dog is smart enough not to keep advancing while it is getting shot repeatedly. And I would suggest looking at your encounter variety to make sure you've got a good mix of creature types and numbers. Whenever the party can settle into one tactic and make it work, that's usually a sign that your combats need some variety. I would especially look at the terrain you are using. If the party has open lines of sight to all enemies and can retreat as far back as they wish, that suggests they are fighting in a lot of wide open terrain.
At the same time, it is a daily power and one extra basic attack each round shouldn't have that much of an effect, particularly if the rest of the party is just shielding the guy and therefore doesn't have any output of their own. Are the combats taking more time to resolve?
Finally, keep in mind that doing too much to limit the use of the power might be seen as a bit unfair. If you really feel the power is unbalancing - and I'm not sure I agree - then your best bet is to speak to the player and ask him to switch the power. But be absolutely sure the problem is really the power and not just that you've fallen into a rut or a predictable pattern without realizing it. Its very easy to do that Ask my players about the two soldier, one brute, two artillery scenario. I didn't even realize that I was designing every fight around the same basic template.