When the WS says they want to be just friends, your answer is to say "oh yes we can be friends, maybe we make better friends than we do lovers."
THAT is what you say to them.. You tell them as if you are perfectly happy being friends. No big deal.
NOW.. What your ACTIONS do after you say that is somewhat different... You do NO contacting except for business that HAS to be done. You do NO pursuing. You do NOT hang on the phone. When they contact you, be cordial, but a tad distant. Not mean. You always want to act like you are very very busy and when they call, you are "right in the middle of something" and can't talk. Get down to the reason they call and then YOU end the call first. Politely. "Talk to you later"
You do NOT tell a WS that "I can't be your friend".. Why? Because it comes across as needy and "if I can't have my way, then we won't be friends. It HURTS your chances. It isn't a strong statement even though some think it is. It IS NOT.
Let them THINK you are fine being friends, and then ACT like they are a "casual" friend that you only talk to when you run into them...
GET IT? Don't cut off your nose to spite your face..