We made it a point to never answer a mishullach after 9:30 p.m. I would tell them (through the door) that it was too late at night. I very rarely gave to meshullachim collecting for out of town yeshivot. I explained to them that we decided what tzedaka we had would support our local institutions. I would say this nicely, but firmly. Of course, if it was hot out, I would always offer them a cold drink.
Besides, we had a dog, so many meshullachim never wanted to come inside anyway. (She’s actually very friendly, but they didn’t know that.)