Sorry to be a few days late to this discussion, but I have an idea that might work for you.
Step 1
Open up the submit_step_2.tpl template file. Put the following on line 1, replacing the domain name with the domain(s) that you want to only allow them to post.
Code:
{if $url_short neq "http://domain.com" || $url_short neq "http://www.domain.com"} Step 2
At the bottom of the same submit_step_2.tpl template file, place the following. You can substitute the error message with your own HTML.
Code:
{else}
Sorry, but you are trying to submit a domain that we do not accept. Please go back and try again.
{/if} Result
The end result is that users will only be able to submit stories to the domains that you put in that {if} statement. If they try to submit a domain other than the ones specified they will see the error message letting them know that they are not submitting a supported URL.
I use a similar method to alter what data appears on the Cuteaholic.com submission pages. If they are posting a video URL it shows that the video will automatically embed itself. If they don't post a known video domain URL it will give them an upload field to attach an image to the post.