Primero ¿Por qué? A menos que haya una alta probabilidad de que esto suceda (en cuyo caso, debería consultar a su abogado ahora de todos modos, parece que está sufriendo el síndrome I-am-afraid-someone-will-take-from-me .
Con toda seriedad, ¿qué le preocupa y por qué alguien compraría su código de otra persona si es gratis? Y si el valor agregado de la tercera parte es suficiente para justificar la compra, ¿por qué querría detenerlos? En otras palabras, eso sería algo bueno para usted (ecosistema más grande), no algo malo.
Respuesta real aquí:
Le sugiero que tome la siguiente licencia BSD y agregue una cuarta viñeta que dice algo como esto.
* Redistribution of this software in source or binary forms shall be free
of all charges or fees to the recipient of this software.
Sin embargo, no soy un abogado, por lo que le sugiero que lo haga por un buen abogado.
Copyright (c) <year>, <copyright holder>
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:
* Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
* Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
* Neither the name of the <organization> nor the
names of its contributors may be used to endorse or promote products
derived from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT HOLDER> BE LIABLE FOR ANY
DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.