Sometimes you simply don't want to go further with someone. Or you feel someone is invading your space.
There can be such a diversity of situations where you have to learn to say "no".
This is the art of building respectful boundaries. You want people to respect your space and specially respect a "no, thank you" sign.
To do that, you need to be firm. You can be diplomatic and respectful but firm. That's by far the best option. If the person you deal with is obviously a space invader and does not respect your limits at all, you can of course show more power and determination in your words, specially if they keep insisting. The key is to repeat the message until the person gets it. Don't give them space to move forward.
If you need to reject someone you know and feel the person very open and vulnerable, you obviously don't want to hurt them.
The trick is to be open and empower the friendship and fun rather than the intimacy. If you feel they want to be intimate with you but you don't want to go that way, the best is simply not to respond to the invitation.
You empower the friendship and fun rather than the intimacy. You don't block, you simply express it differently. What hurts is the blocking of emotions and energies. Now, you can truly transform an exchange into something friendly rather than intimate if you want to.
That in itself validates the other person and offers them a new space for exchanging.
In a way, you don't reject, you simply respond in a different way. This move is a win-win for both, because you open a new door, a new space. You stay respectful without hurting anyone's feelings.
To shift from an "intimate" to a "friendly" mind set, you need to empower the fun and lightness of the moment.
At the end everyone wins from clear boundaries. Strangely enough these boundaries do protect intimacy. They give you the power to go for it when you want to.