I am using a 10CL006ZU256I7G FPGA. “I don’t need to use all of the banks in the U256 package. What is the most power efficient way to not use an entire bank?”

Hi, You can use the Quartus setting Quartus II software -> assignments -> settings -> devices -> unused pins. To make the unused pin connection. Better to connect to gnd. And also follow the recommendation from pin connection guideline https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/dp/cyclone-10/pcg-01021.pdf