What did they mean when they announced that "No man or woman should do any more Melachah..."?
Ramban and Moshav Zekenim: It incorporated both not bringing 1 any more donations 2 and the women not spinning any more goats'-hair - meaning that all activity relevant to the previous command should cease.
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.
Bechor Shor: This teaches us that carrying from the R'shus ha'Rabim to the R'shus ha'Yachid is a Melachah. Refer to 36:6:2:2.
Money (property) is called "Melachah", as we find in Bereishis 33:14 and in Sh'mos 22:7 (Ramban).
Why did Moshe issue an announcement that they should stop all work for the Mishkan?
Ramban: Because they had more than enough for its needs.
Why does the Torah insert the unusual word "va'Yikalei ha'Am me'Havi"?
Ba'al ha'Turim: Because it inserts the same word regarding the flood in No'ach Bereishis, 8:2 "Vayikalei ha'Geshem". To teach us that, when the clouds finished bringing [the gems] the people stopped bringing their donations.