How to block baseboard (water heated)?
Since this seems to be an ongoing problem for you, I might suggest a somewhat more formal approach to it. For instance, use magnets to attach toweling to the top of the radiator shell and let the duble-thickness of that towelling (magnet inside the fold) drape all the way to the floor - that should basically shut-down air flow through the radiator from top and bottom and be a bit neater besides. If you need to step it up a notch from there, remove all those, unroll a sheet of aluminum foil from one end to the other, and replace the towel over the top of that.

In both cases, do not try to "stuff things into" the radiator shell, but rather, cover them over completely from the floor to the top-exterior of the radiator shell, all the way to the end-caps (don't limit yourself to the "active area" with fins.)

If you have to buy supplies for this, the roll of aluminum foil would be cheaper to start with than a bunch of towels. Flannel sheets or whatever is cheap in the fabric store may be cheaper than towels as a source of long strip fabric.