Tell them you use a keyboard multiplexor (that is what KeyClone is, that is what a Vectra is, that is what 1 wireless keyboard with 5 receivers is) and that you press a key and it goes to all 5 accounts.

You aren't lying.
You aren't giving them ammunition about running 3rd party software.
You aren't giving them more information than they can understand easily.

Keep the following in mind:
My guild, Band of the Black Hand, had it's name forcibly changed by GM's due to the name violating the naming convention (It had "Black Hand" in it which was too close to "Blackhand". For 8 months I petitioned once a month appealing the decision. For 7 of those months and multiple contacts with GM's a month I was told that the name was a violation and should have been changed. The 8th month they restored the name.

On a similar note, a friend was using 3 accounts for real money trade, he was mining the auctionhouse and making enough gold that it was $14/hour US... they banned all three accounts. He petitioned the GM's over and over as well, and eventually got two of the three accounts restored.

You have to catch them on a good day in a good mood for that sorta results, but you catch the wrong gm on the wrong day in the wrong mood, admitting openly to using software could get you a banstick. It's not likely, especially considering the multiple blue posts.