I feel Caveat Emptor, let the buyer beware, applies strongly here, but I don't see a problem with an SP continuing to provide services to someone who may be developing deeper feelings than would be healthy for them. Really it's a problem by and for the client and whether or not someone wants to tackle that or not is a personal choice and not an ethical obligation. This might be akin to a bartender refusing service to someone who's drunk, or where someone has developed a possessive/jealous/stalkery streak, which is again, not the SPs fault.
Of course, if someone is crossing into actual manipulation that harms someone by dangling more in order to, in effect, rob someone under false pretenses, than that's wrong (I don't mean, act pleasant so they come back, that's just good service and business, I mean like "I think I love you and you should trust me, what's your PIN number?") I think at that point whoever is manipulating isn't an SP, they are a criminal con artist, and those exist everywhere, across all genders and fields.
I don't think an SP should be any more responsible for their clients well being or mental health then any other service or sale industry. I used to work at the Liquor Commission and, while I had a legal responsibility to not sell to someone already drunk, I didn't have an obligation to refuse to sell and then sit an obvious alcoholic down and explain how drinking is ruining their life. Restaurant servers don't have a responsibility to refuse a pizza to an obese man with a possible heart condition and car salesman aren't obligated to refuse to sell someone a Ferrari and sit them down to explain that having a small penis is totally fine.
TLDR: criminals are gonna crime, clients are responsible for their own feelings, and SPs are not any more required to take responsibility for someone else's financial, physical and/or emotional well being.