How does a gentleman’s daughter refuse a marriage proposal?

We have Elizabeth, she of the quick tongue and wit, whose manner is so charming she hardly offends.

However, when Mr. Collins announces his intentions to propose, Elizabeth had time for nothing but “a blush of surprise” before her mother answered Collins. Did Elizabeth not have an idea this was coming? She certainly knows what is to follow after Collins announced his intention of a private audience with her, “but is sensible that it would be wisest to get it over as soon and as quietly as possible” so she sits down with feelings divided between distress and diversion.

However, once Mr. Collins has launched himself on his business of proposing to her, his speech “made Elizabeth so near laughing that she could not use the short pause he allowed in any attempt to stop him farther...” This gives me the idea that it would have been appropriate for a woman to stop a gentleman from going further in his proposal if she did not intend to accept it, just as she would have been correct to discourage a gentleman from offering a proposal that she did not look for. (Sorry I could not find a specific reference to this in my search of L&T and previous GR archives, but I am convinced I read these things at RoP!) I do not mean to imply that Collins would have been sensible enough to stop, but I had thought a lady should at least try to “minimize the damage” as it were.

Elizabeth’s second proposal of marriage catches her completely off guard. She is fairly wallowing in her dislike of Mr. Darcy after re-reading Jane’s letters: Elizabeth was surprised, but said not a word. After a silence of several minutes, he came towards her in an agitated manner, and thus began --
"In vain have I struggled. It will not do. My feelings will not be repressed. You must allow me to tell you how ardently I admire and love you."
Elizabeth's astonishment was beyond expression. She stared, coloured, doubted, and was silent. This he considered sufficient encouragement; and the avowal of all that he felt, and had long felt for her, immediately followed.

When Mr. Collins was proposing, Elizabeth was distressed, but also diverted, and seemed not to be able to say anything to stop him because she was busy trying to keep from laughing at him, which was most civil of her. However, when Mr. Darcy begins his proposal, she is not at all diverted, and was astonished beyond expression and remains silent. I can not recollect another instance where Elizabeth can find no words equal to the situation. Had she been able to get out as little as a harshly spoken “Mr. Darcy!”, doesn’t it seem that he would not have felt sufficiently encouraged? I realize our plot would suffer, but I wonder at our well-bred young lady being insufficiently motivated to stop the gentleman from greater mortification than the four sentences he has already uttered expressing feelings that she cannot return.