Hmm, I could not even find how to disable resume in docs from MS. Are you uploading files or downloading? FTP resume will only work for download with IIS.
I have verified that resume is supported on Plesk-controlled server using Unix command-line FTP client:
...
ftp> rest 100
restarting at 100. execute get, put or append to initiate transfer
ftp> get index.html
200 PORT command successful.
350 Restarting at 100.
150 Opening ASCII mode data connection for index.html(557 bytes).
226 Transfer complete.
457 bytes received in 0 seconds (457 bytes/s)