you could configure your firewall to be open on port 9982 only to known IPs (i.e. those you want to support). That way, no one else could upload to your host.
This does seem rather pointless though, since data that is uploaded is currently made 6x redundant on the Sia network. Also, files are split in pieces and distributed across hosts. This is all to ensure that even if a significant number of hosts go down data is still safe
Meaning, your preferred partner still have to upload to multiple other hosts besides yours. In fact, your host may not even be selected by the Sia client running on your preferred partners computer as host selection is random, though weighted by published price.
I think that what you are requesting is not really compatible with the Sia protocol and if enforced would reduced the networks reliability overall.
If you wanted to support just one uploader, you would be better simply using rscync etc.