That is probably a 'feature' in some very old MLDonkey clients. Works fine with other MLDonkeys, I suppose, but rather annoys everyone else. The newer versions don't seem to have this.
If he has the same IP address all along, (static IP), you can put that IP address in your IPfilter.dat (open the file in your config folder with Wordpad and add the entry in the proper format).
<IP address>-<IP address>,<level>,<comment>
Use the same IP address twice (the format is for a range of IPs).
Now you shouldn't see him at all (after a restart), but that means you won't download from him either.