I tested the command on Linux and it seems to work as expected, so I
don't know if this is a macvim specific issue. In normal mode CTRL-C
aborts the pending command, but from :help map-CTRL-C,
"Using CTRL-C in the {lhs} is possible, but it will only work when Vim
is waiting for a key, not when Vim is busy with something. When Vim is
busy CTRL-C interrupts/breaks the command. When using the GUI version on
MS-Windows CTRL-C can be mapped to allow a Copy command to the
clipboard. Use CTRL-Break to interrupt Vim.",
so it should work. Have you tried it in Vim from the terminal?
Regards,
-- Aymen --