Moshe had asked the people to contribute to the construction of the Mishkan. The people were so generous that more contributions than were needed came in. Moshe then told the people that there was enough to finish the construction and more contributions were not needed.
The language Moshe used to tell them was somewhat dubious. He asked that an announcement be made: אִישׁ וְאִשָּׁה אַל יַעֲשׂוּ עוֹד מְלָאכָה לִתְרוּמַת הַקֹּדֶשׁ, “…Man and woman shall not do more work toward the gift for the Sanctuary.” (Ex. 36,6) Why did he not simply say that they should stop bringing their contributions for they were no longer needed?
Perhaps the reason was that we must never imply that contributions are not needed. You can say that for a particular cause no more contributions are needed. You should never express yourself in such a manner as to tell people to stop giving.
Parents must impress upon children the importance of giving donations and contributions for all good causes. Rabbis and leaders of organizations must constantly influence their followers to donate and stress the need. Moshe did not tell them to stop giving. He told them that there was no longer a need for their contributions towards construction of the Mishkan.