A write-only rsync server can be used by anyone to upload content with no risk of deleting existing files. Assuming access to the rsync server is handled via ssh, the following line can be added to the ~/.ssh/authorized_keys file
command="rrsync /usr/share/nginx/html" ssh-rsa AAAAB3NzaC1y...
The rrsync script is found in the rsync package documentation and installed with:
gzip -d < /usr/share/doc/rsync/scripts/rrsync.gz > /usr/bin/rrsync chmod +x /usr/bin/rrsync