You can use either the header option or add the "filter-false" class to the TH tag for that column.
The header option is documented in the tablesorter documentation - click on "headers" in the property column of the configuration table. The syntax for the option is:
headers: { 0: { filter: false} }
Use the following CSS if you don't want to show the default disabled filter formatting:
.tablesorter thead .disabled {display: none}