" A snowflake is an apt metaphor for a spinster, a woman separate and unattached, not moved or controlled by another person (and particularly not by a man), a snowflake drifting. In the fourth stanza the girl definitely feels tempted by "a burgeoning" of sexual desire that affects her physical senses ("her five queenly wits"). "Vulgar motley" represents allowing sexual feelings to take over uncontrolled. The girl sees this state as "A treason not to be borne" or her body working against her rationality. Society (and her mother, perhaps) has told her only fools allow men to have their way, that women who have sex before marriage become society's pariahs. So the girl says, "let idiots reel giddy in bedlam spring; she withdrew neatly." Bedlam spring is unrestricted love. She doesn't dare participate, and so she withdraws from the man.

In the final...

"And round her house" (house is a metaphor for her body and consciousness) "she set such a barricade of barb and check" (barb and check are controls such as unfriendliness, a critical attitude, sarcastic comments, etc.) "against mutinous weather" (uncontrolled passion) "as no mere insurgent man" (someone attempting to break down her defenses) "could hope to break with curse, fist, threat or love, either." This girl is so uptight and afraid of sex, she is unable to recognize love as anything but a threat to her control over herself.
It's not that she is incapable of feelings but that she chooses never to express them. There may be other reasons why this particular woman never wants to get married, but her attitude towards sexuality is the primary focus in the poem.