The only way to prevent this from happening, either this year or in future years, is for the voting public to inform their Senators and Representatives loud and clear that they do not want any such memorial created by the federal government, whether on the Washington Mall or anywhere else.
Among the pious cant that we are being fed by those pushing this proposal is that a memorial will serve to remind future Americans that slavery was cruel and evil. Most Americans understood that in the 19th century!
What a memorial would do is perpetuate the fraud that slavery was something peculiar to the United States, when in fact it was one of the oldest and most widespread of all human institutions, existing for thousands of years on every inhabited continent, involving people of every race and color as both slaves and slaveowners. Even in the United States, there were thousands of black slaveowners, and in Africa many more.
The United States was one of many Western nations which turned against slavery in the 19th century -- while non-Western nations bitterly resisted efforts by the West to get them to abolish slavery. Only the fact that Western imperialists had more firepower enabled their revulsion against slavery to prevail.
Maybe we should have a monument to historical truth somewhere, though Washington hardly seems the place for it.