Learn Turkish: Arılar sokmasın diye kalın kıyafetler giyiyorlar. - They are wearing thick clothes so that the bees do not sting.

In Turkish, you create a purpose clause ("so that...") by combining the 3rd person imperative (command) form of a verb with diye. Literally, the sentence is structured like: "Let the bees not sting, they say, and they wear thick clothes." Here, sokmasın is the negative imperative of sokmak.

Since arılar (bees) is plural, shouldn't it be sokmasınlar?

You could say sokmasınlar, but it's completely optional and usually left out. In Turkish, when a non-human plural subject like arılar is explicitly stated, the verb usually stays in the singular form (sokmasın).

Doesn't the verb sokmak usually mean "to put in" or "insert"?

Yes, that is its primary meaning! But in Turkish, insect stings and snake bites are always expressed with sokmak. You can easily remember this by picturing the bee "inserting" its stinger.

Could I use için instead of diye to express "so that"?

You could, but the grammar would become more complicated. To use için here, you would need a verbal noun with possessive endings: Arıların sokmaması için. Using the imperative + diye is a much more natural and conversational way to express this!