Can't empty Trash because an item is in use

I can’t empty the Trash on a Mac running High Sierra, despite having tried all the usual methods.

There are only two files in Trash: “Backups.backupd” and “LittleMac” (both with many subfolders).

When I try to empty Trash, I get “The operation can’t be completed because the item “foo” is in use”

(where “foo” is one of the subfolders).

When I select a folder and press Command-Option-delete, either the folder disappears then immediately reappears, or I get the message “The operation can’t be completed because the item “foo” is in use” (where “foo” is the name of some subfolder of the one I selected).

Control-click and “Delete Immediately…” produces the same result.

I have tried restarting, and it makes no difference.

I have tried dragging the items to the Desktop so I could Command-Option-delete them from there, but dragging them performs a copy rather than a move, so I’m still left with the original in Trash.

None of the folders have any contents except subfolders. They are all size zero or 10 KB.

I have read and write permission on every folder and subfolder.

The folders are what’s left of Time Machine backups, but Time Machine is no longer configured.

That is, Time Machine is not running, and has no backup disk selected.

I have no applications running, and none are hidden.

I have tried running “lsof ~/.Trash” and it lists nothing (except “.” and “..”) in the directory. (Just to check, I tried moving another file into the Trash, and "lsof ~/.Trash" did find that one.)

Any suggestions?