You have suggested a simple, effective and even common sense idea. I don't see how it could be abused as indicated above.
One of the main things which needs to also happen, is that the job ads need to be screened first before going live. If a 'repeat offender' (so to speak) posts an ad then their name should flag up to that Administrator. I would screen the ads myself If I had to, it's really annoying.
Sorry about your predicament, Melanie. Never start working without an official contract and never send free samples so easily.
Make sure that they have a verified payment method before starting work.
If they wish to see your writing capability, then add samples to your portfolio and direct them there.
You could try a site like My Ows to protect content which you've written. As far as I'm concerned, you haven't been paid for your work so it's still yours.
All the best.
Melanie, please forgive me if I have this wrong, but you don't appear to have any 'working' jobs on your profile page. I know that this can be a glitch, because I have a small job I am working on and it is not appearing on my page either.
However, there are three basic things to remember when freelancing from this sort of platform:
1) Read everything there is to know about it including the Terms of Service.
2) Never do free tests. I know that oDesk/Upwork's attitude on this is pretty ambivalent (although it is against their ToS), but 8 times out of 10 you will be burned if you do.
3) If your transactions were carried out via Skype and off-Upwork then you were really violating Upwork's terms of service and if you had carefully read them, you could have avoided all this. (Correct me if I am wrong.)
I doubt you will be paid. However the upside to that is that the work you have done remains yours, and you can check on Google to see if the 'buyer' has used them.
UpWork scam - **edited for Community Guidelines**
WARNING: DO NOT FALL FOR ONE OF THESE JOB OFFERS OR YOU COULD LAND
The following is a partial list of Fake Jobs (Money Mules). These
are jobs that ask you to act as their money transfer agent. To be a
money transfer agent requires you to be approved by the Treasury
Department, licensed, bonded and insured:
**edited for Community Guidelines**