I have seen that done, basically a company I worked for had a contractual agreement to use a certain agency for their contractors. The agency finds out contractors are being hired and they are not being offered the opportunity to out forward candidates. The recruiter came to a meeting with my manager and was very cordial and asked why they were not being used, the manager said he didnt like them (or something similar, basically making it a personal issue with them). The recruiter simply said they had a contractual agreement and whilst they probably would not pursue it legally they would quite happily use his company as a resource to select contractors from.

After 2 staff members were offered new positions the manager decided he didnt hate the agency that much. Whilst I liked the manager it was refreshing to see someone combat this situation whilst expressing clearly they would not pursue it legally.