I'm surprised that duct taping didn't work. I find that tying the grasses with a twine lasso works quite well. It helps to have two working on the project. One can cut while the other holds and takes away the tied grass bundle.

I used an electric trimmer on the grasses around my home, which included at least 2 dozen Pennisetum Rubrum clumps. They cut down rather easily and I had neat bundles to haul away. It's hard to avoid some cuttings getting into the mulch, but with rock mulch, I'd use a blower to clear away debris. More in question for me is whether my Rubrum will come back after the couple of freezes we had here in zone 8b this year.

Here's a link to a good explanation of the lasso procedure.

Here is a link that might be useful: Ornamental Grass Maintenance