I see that you've just joined ENworld, so perhaps you're not familiar with the way discussions here are typically held.
An individual (such a yourself) postulates a theory about why something should work a certain way. Then others post either (a) opposing theories that produce the same result, or (b) reasons why the initial theory is invalid or wouldn't work for some reason.
In general, someone who posts about how they would like to add a rule that isn't in the books (damage to individual eye stalks, sundering the limbs of an octopus, poking a giant in the eye with a spear) are usually told that in their game they're welcome to introduce any rule that they feel is appropriate. Such answers usually include rule quotes (or mention of rules) that describe why this shouldn't be done, but it's your game so do what you want.
In the case of beholder eye stalks, there are no rules that allow you to target individual eye stalks whether it's a melee attack, ranged attack, or area of effect. Therefore what you're postulating is not possible using the 3.x rule set. If you badly want the ability, add it to your game as a house rule.
As a general piece of advice, be careful about what rules you add. It's very easy to go down a slippery slope where a single rule change in one place means that other places should have similar rule changes. And pretty soon you have too many house rules to keep track of. Examples of this include the massive damage rule (and house rules that tweak the way it works), falling damage, and anything that is magical or magic-related related.
Good gaming!