We know that there is a Melachah (prohibited labor) called Tzidah. What is Tzidah? Tzidah, Rabotai (my masters), is trapping. You know how a lot of trap, Tzidah? Okay, Latzud (to trap). Now, Rabotai, there are different levels of Issur (prohibition) of Tzidah. Something that is generally trapped: if a person traps it on Shabbat, he's chayav (liable), chayav chatat (sin offering). Okay, the things that are not generally trapped: if he traps those things that are not generally trapped, it's not the derech (customary way) to trap such things. So it's Asur Miderabbanan (prohibited by rabbinic law).
About flies, the discussion concerns trapping them. In such a case, if we trapped it, bees too are things that are generally trapped. You don't want bees, so you trap them. Some rabbis say no, only the Lebanon. In any event, you're not allowed to trap a fly. So what does it mean to trap a fly? You see a fly coming. Now it's on the table. It's annoying you, so you take a cup and you just put it on top of the fly. That, Rabotai, is asur (prohibited), asur. Why? That's tzad (trapping), and therefore a person cannot do such things. Be'ezrat Hashem, we'll continue with these halachot (laws).