Or, What to do when an unsuitable yet importunate suitor comes courting. It's another tragic, doomed romance. There is no tentacle pr0n here.
(I have no idea what those are and couldn't find them, but why let that stop me?)
Seven arts of self-defense
Neville was out on the Lake one day, rowing to a spot where he thought he'd seen an unusual variety of Gillyweed in passing when, without any warning at all, he was forcefully dragged into the water. His arms were pinned to his sides by an irresistable grip, not that he could think of a spell that would be effective against... the Giant Squid.
It flexed the tentacle around him gently, and rolled a Giant Orb at him in a way that might have been called liquid and affectionate if the Giant Squid had been a Giant Puppy or Giant Calf.
1. Playing dead
Neville played dead. His eyes rolled up, showing the whites, and he went limp and unresisting. Actually, he passed out.
The Squid did not care and happily carried him through the water to its lair.
When it gently set him down in an enormous underground stone cave and turned away for a moment, Neville quickly cast a Disillusionment and tried to blend in with the surroundings. The Squid turned around, and not noticing him, waved its tentacles in confusion and smacked him back into the water with a huge splash. It hastily fished him out and looked very apologetic.
3. Lie like a rug
"The truth is, I already have a gir-"
Neville thought it was prudent to stop at the dangerous glint that appeared in the Giant Eye.
4. Polite rejection (the Destiny card)
"You know it isn't possible - you belong to the Lake, and I can't live underwater. And my parents would never approve. It just isn't meant to be."
The Squid waved its tentacles around the huge stone cave, at a Giant, fishy-smelling pile of Gillyweed, and generally seemed to express the idea that it wanted Neville to forget his parents and run away with it.
"Oh, look at the time! Can we talk about this later? I have to go for my Potions class. I love Potions! Professor Snape is such a wonderful teacher..."
Even the Squid looked rather sceptical at Neville's failing show of enthusiasm.
"I don't know what you see in me. Surely I'm not good enough for someone like you. I'm not famous like Harry, smart like Hermione or good-looking like..." Actually, he was beginning to depress himself. The Squid stroked his cheek gently, making it clear it didn't care about all the other people.
7. Kamikaze attack
Steeling himself, Neville announced, "I have a secret. I really, really like fried calamari rings."
The Squid turned many different colours and the tentacle around Neville tightened dangerously as it shuddered in horror. It lifted him up, and Neville flinched, expecting to be smashed against a hard surface.
It slowly lowered him, looking heartbroken. The Giant Squid carried him back to his boat, then silently sank beneath the dark waters of the lake.