I've noticed a former potential partner of mine is using a logo, website design, and company description/mission that I created and retain full ownership of.
This IP is pretty basic: $750 cost to create.
As several people commented, $750 is not worth going the legal way, certainly when you have no way to prove it.
While I can see how this could be irritating, Life's too short to spend time on this.Move on, and focus on your current endeavors.
I'd talk this one through with a lawyer if I were you. The problem with sending the letter yourself is that you're potentially setting yourself up for an action for a "declaratory judgment" from your former partner. I'd go through your records to see what you can prove about who developed what and show that to the attorney -- it'd be a shame to pick a fight only to find out that your ex-partner is legally a "joint owner" that can do whatever he or she wants.
If you created the idea while the partnership existed, and then the partnership broke up later, he may rightfully have just as much claim to the IP as you do, depending on the legal form of the partnership and any agreements the two of you had.
Have your lawyer send a cease and desist letter. This usually does the trick but be ready to follow through if you go down this road.
It is not that difficult to have his web site removed from the web, provided you can prove ownership and don't mind a possible legal battle latter. I'm not sure if it is worth it for $750. He can just get mad and sue you. Your resulting legal costs will well exceed that $750 figure.
However, if you want to have his web site removed from the web (and you meet the requirements for doing so), see my answer here at OnStartUps:
It is not clear in your message is he 'former potential partner of mine' or 'former partner'.
If he is real former partner and you did not have any legal agreements why he can not claim the same rights on this as you do. Or he can claim the right on some other stuff you developed being together. In this case you probably can talk with him about IP ownership basing on contributions into that stuff. Sure thing he may not listen you.
He is your competitor and it is already a fight. If IP is worth 750$ it may be a waste of time to claim it. Just be better, provide new services and features, re-design your site etc.