Can someone please tell me how to shut off a Blackberry 9800T phone that is locked? I don't want to shut-off the "locked" feature, just the cell phone itself. (It's not mine and I don't know how to use it nor have the password).

Remove the battery.

You might see if the carrier tag is on the display or marked and call them to report the phone you found.
They "might" be willing to contact the owner