One way to approach this could be to forget "no strength" for now, and focus with them on "less effort" . Often you can show people how if they do technique more correctly it will cost them less effort. And that gets the beginners working in at least the right direction. Throw in a joke about how aikido is for lazy people and you're good to go.
I like this answer. "No strength" is something that even fairly advanced people struggle with. "Less effort" is easier for a beginner to understand, is intuitively desirable, and is more likely to give good results. (Which for beginners means "uke falls down.") And puts people on the right track: keep reducing effort, and eventually you get to zero.