How do I explain that mana weaving is cheating

One of the people at the LGS I work at got called out for mana weaving, debate as to whether or not it was cheating got heated because apparently another player at the table also did it, and eventually I got called off the desk to help.

I'm really not good at rule-speak, so I only managed to calm it down by basically insinuating that if the group didn't quiet down I was going to kick them out.

For my future reference; how is the best way to clearly explain that mana weaving is cheating?