T-Bills are tracked via the book entry system. I.e., the Treasury keeps a registry of who owns what debt. So all we need to do is repudiate all the debt that China currently owns. Then there will be no buyers.
At which point every other holder of U.S. Treasuries will wonder "Am I next?". This will lead to a massive sales of U.S. debt instruments around the world, and would not be good for anyone.
Now UBS will worry we will do the same to them, perhaps in response to Switzerland becoming hostile? Nomura dumps our bonds in anticipation of Japan going all 1942 on us?