Hello,
I've been teaching in Shanghai for the past two years. I encountered that problem quite often when I taught older students. The method I found most effective was to clap the number of syllables in the word you're drilling.
For example, I had a student named Jack, and his classmates would call him Jack-a. We'd clap once and say Jack at the same time. Then we'd compare that to Jack (clap) a (hands apart). I would be really theatric about hands separating on the "a". For the most part, my students would laugh and understand. If they continued to add "a" to other words, we'd repeat the process.
I hope this is helpful!
