What did they mean when they announced that "No man or woman should do any more Melachah..."?
Rashbam: They meant that the women, who spun the goats-hair at home, should cease doing so.
Seforno: They meant (not that they should not bring any more donations, but) that they should not do any more work for the Mishkan, such as spinning the goats-hair, making the wooden planks and preparing the hides.
Why does it say "va'Yikalei ha'Am me'Havi"?
Ba'al ha'Turim: It says so also about the flood "va'Yikalei ha'Geshem" (Bereishis 8:2). We learn that when the clouds finished bringing [the gems, also] the people ceased bringing.