Just explain them in black and white that the assignment/home work is like a test/exam where teacher want to judge the individual student's effort/approach/understanding and this should be coming from the student him/herself... so if they borrow your effort that means its like deceiving the judgement of the teacher or cheating and this is not GOOOOOD.

At the same time you should always assist others in guiding on how to approach/response the assignment...this actually will improve your performance and knowledge....

Hope this helps

Good Luck