Why? It makes no sense to restrict the ciphers on the client side.
If they want their clients to use specific ciphers, they should configure their server to require those ciphers.
Anyway, you can configure WinSCP to warn, if it tries to use specific ciphers. See https://winscp.net/eng/docs/ui_login_ssh

Is it possible to remove a cipher from the list of ciphers to use on a SFTP connection? I have a vendor that would like us to remove blowfish and 3des from the list of available ciphers.

Thanks!