1)Under Options/Spam Management, just set it to: EDIT: Be sure and see Xon's suggestion below.
2)Create a Notice saying:
Do not post a link -or- email address until you have made three posts, as it will be rejected as SPAM.
with the User Criteria being User has Posted No More than _2_ Messages.
This way, users who have not yet posted at least 2 messages will SEE this Notice and be aware of such. If they choose to go on and attempt to solicit others (before they have made 2 posts), then the post is automatically put into the Moderation que.
Lastly, another method is to use @Xon's Crowd Moderation add-on so all other members can help you keep an eye out for anything that you consider unwanted solicitations.