It's OK if you don't feel the same way you did before, but let your hook-up know as soon as possible.
Even if you haven't been hooking up for that long, it's important meeting in person if you decide to call it quits.
Instead, choose a moment when neither of you will be distracted from the chat.
"I've really enjoyed our time together, but I don't see this relationship going anywhere," you may add.
"I" comments are totally focused on your emotions and experiences, with no blame placed on the other person.
If the relationship is really informal, a simple text may end things while still giving the other person closure.
When you and your hook-up have opposing long-term objectives for the relationship, it may be quite difficult.
There's nothing wrong with unfriending, unfollowing, or even banning them to allow yourself some breathing room and solitude.