I heard both sides from different people. On the one hand, there is a halacha that one may not turn an oni away emptyhanded. This applies even if you have already spent all of your ma’aser money. Yet, a peruta suffices. However, giving a dime to a collector shames him more than not answering the door. Therefore, I heard from a most respected posek that in such a situation one should open the door, and with respect explain that one cannot give at this time, while offering the person a drink, or a snack, something to alleviate the embarrassment inherent in collecting. Even then, not turning someone away only applies to a person collecting for himself – the oni, and not a gabbai tzedaka or a collector for a yeshiva.
The second side is that giving everyone a dollar does very little, while giving let’s say all 25 to one individual carries some weight in his situation. Should one, thus, pick those whom he considers in greater need, denying to the rest. If the rest are personally poor, you have the issur of turning an oni away. We aren’t obligated to personally fully provide to any individual, while we are forbidden from not giving to real aniyim.
Therefore, giving each a dollar, explaining that you simply cannot afford more at this time, and doing so besever panim yafos, is the best way to go about it. After all, a person collecting, due to his own state, realizes that not everyone has, and will not be offended if less is given, if it is given nicely.