Does anyone know what happens if you let go while a portkey is transporting you somewhere? Is it like Apparation, where you end up splinching? Or like the Floo network, where you end up in some place that you didn't want to be? Or is it something entirely different?
In Harry Potter and the Goblet of Fire, in the chapter "The Portkey", the feeling of being transported by a portkey is described as ...
It happened immediately: Harry felt as though a hook just behind his navel had been suddenly jerked irresistibly forwards. His feet had left the ground; he could feel Ron and Hermione on either side of him, their shoulders banging into his; they were all speeding forwards in a howl of wind and swirling colour; his forefinger was stuck to the boot as though it was pulling him magnetically onwards and then [...]
So it's probably either physically or psychologically impossible to let go of the portkey.
The obvious next question is: What happens if someone cuts off your finger that connects you to the port key?
The first of JK Rowling's WOMBAT tests had a question asking to identify which of a set of "commonly held wizarding beliefs" was true.
Question 13 out of 17
Which of the following commonly held wizarding beliefs is actually true?
☐ If an inanimate object appears to think for itself, Dark Magic has been involved in its creation
☐ The use of magic in front a Muggle is prohibited unless the witch or wizard is under threat of personal injury.
☐ Releasing a Portkey before it has arrived will result in death or serious injury.
☐ 'Finite Incantatem' should be used as a precaution when a Muggle rings the doorbell.
☐ Bad luck can be prevented by turning three times on the spot and deliberately Splinching one's thumbs
W.O.M.B.A.T. Grade One - Question 13
One of the choices was "Releasing a Portkey before it has arrived will result in death or serious injury.", but no credit was given for that choice.
Thus we can presume that letting go of a portkey (if even possible) wouldn't result in any serious harm.
You cannot let go of a portkey; you actually do not hold it. They are only touched ; therefore a magical force, the powerful spell is what holds you to it. That spell does not allow anyone to deviate from the destination.