In "The Hundredth Smurf" (third story in the "The Egg and the Smurfs" album), a hundred Smurfs are required for the Dance of the Moon, but there are 99 only. In "The Wild Smurf", we find that the Delivery Stork lost a Smurf who ended being the eponymous character. This means that if Wild Smurf didn't have get lost, they would have been 100 Smurfs! It fits!
In The Movie, when the Smurfs go to an antique book shop to get the remaining elements of the potion required to go back to their world, they come across a book about their species. One of the pages from the original books shown during this scene comes from the book "The Smurf Apprentice", in which a relatively clumsy Smurf tries his hand at magic. ...a clumsy Smurf. Now, who is one of the main Smurfs in the movie?
The Movie sheds some light on Gargamel's entire motivation for capturing Smurfs - their skin flakes, sweat and tears carry potent magic, and a sample of Smurfette's hair allows him to power his ring and a wand. Surely the Smurfs can reach a compromise, like Gargamel leaving their village alone in exchange for weekly bags of shavings and hair clippings, toenail clippings and even tubs of bathwater...
To be fair, Gargamel has the Villain Ball super glued to his hands.
More importantly, it is a standard principle of magic that things like your hair, nail clippings, etc. can be used against you if a suitably evil magician were to get his or her hands on them. Very likely the Smurfs would be extremely wary of voluntarily giving those things to Gargamel, who fits the profile of "evil magician" to a "T".
Not to mention the movie states that just a tiny portion of Smurfs essence has made Gargamel a serious threat to the Smurfs. If the Smurfs were to give him their hair, and such things that were extracted from Papa Smurf in the movie, Gargamel would go to next step... and try to (of course) take over the world. Or at least, the known world at that time.