How can I create a user account that only allows for uploads via sftp?

I currently have a website with an upload page. We have users who want to upload lots of data (gigabytes), and the website is proving to be fickle. As a result, we're considering letting the users have sftp access to upload data to a dropbox file.

How can we set that up using Ubuntu linux? I'm not a very competent sysadmin, so we've just been locking everything down but the web and ssh/sftp for our own use, then only allowing web access through passwords that only work on the site via the site's database.

I'd think that we'd need to set permissions on a user so that:

1. the user has a home directory.
2. that directory is write-only for them
3. the user can access no other directories on the system
4. the user cannot execute any programs whatsoever
5. The user can, essentially, only upload files via sftp, on the standard ports for that service.

How can I do this? As a side note, the user will be using our own upload client that also collects some domain-specific information, like what the upload is for; I'm not sure that that detail matters, but I mention it because the user will not get to run ls to get a directory listing, so a standard ftp client may not be useful to them.