I would say use the old remedy of two house bricks but with a guinea pig you'd probably squash the poor bugger!
Watched a piglet get de-bollocked on Kill it, Eat it, Cook it not long ago. They just used a scalpel, opened up the sack and lopped them off...
Mr Brain's Faggots anyone?
Used to do the piglets when I worked on a pig farm in the 60's, well I held them while the yokel did 'em. It was dead quick and then you poured that yellow iodine straight into the cut to stave off infection. They were dead 'ard them pigs, a quick squeal and then they ran around as if nothing had happened. I got the job when the Jimmy saw me, asked me where I was from, told me that Birkenhead was way out of pig country but asked did I want the job anyway to which I answered "yup, Sir!" Funny job for a sailor but there you go, it was on a shore station by the way, we didn't keep livestock on ships by then.