It sounds like you're a staffer, and if that's the case then I would encourage you to have the member speak with this person! In my opinion, this is not a task which the staff should have to handle, but then again I place strong emphasis on members being able to stand up for themselves and communicate their wants and needs. If you decide to get involved as a staffer, then in order to perpetuate fairness you will have to oversee any characters that have been created from a wanted ad. That's not necessarily a bad thing, but it depends on how much work you want to put on yourself vs. your members.
But moving on!
Your member should keep this in mind: whether or not they confront this player, there is nothing that forces them to roleplay with this character. There is nothing forcing them to say that their wanted ad has been fulfilled if they don't feel that it has been.
If they want to talk to this player, then there are a few different ways they can approach the situation. Either tell that player the above- that even if the character gets accepted, they will not consider the wanted ad filled and do not want to write that plot with that character; or, be honest and open with the other player and tell them simply that they're not interested in them taking their wanted ad. I recommend the latter over the former, although it should be followed up with the former if the person in question does not relent.
If you decide that staff intervention is necessary, then how you solve the issue is up to you. Some variation of the above can be included in the message you send them, but whether you pend the application or reject it depends on how you want to handle things on your forum.
If the posting claims before the application is accepted is a problem (an issue which I would consider separate from the matter of taking a wanted ad without discussing it with the person who posted the ad first), then I think a simple reminder to the member would be sufficient.
Good luck, Anon!
If you would like to ask me for advice, you can comment on an article or send me a private message! Be sure to include if you would like to remain anonymous.