Gaffer tape. We used something called rhino tape, but there are loads of brands in good DIY shops.
The NHS no longer freezes verrucas off and my GP said it was too risky to try an over the counter freezing remedy on my 4yo son. I was prescribed the salicylic acid gel, which is generally acknowledged to be useless. I used it anyway and watched as my son's verruca got bigger and bigger. I tried an Internet remedy - gaffer tape over the verruca - but my OH didn't think it was a good idea because the doctor hadn't mentioned it so we stopped after a few days, although I could tell it was working. After 11 weeks, with the verruca now looking huge and really protruding from the ball of my son's tiny foot, I went back to another GP. He sympathised and suggested I try the freezing remedy as my son was in pain and had now developed hives on the foot as well.
I was worried about freezing off live tissue by accident so I tried the gaffer tape again, and a week later - no verruca. A pain-free, risk-free non-invasive remedy. There is no need to keep the gaffer tape on so long that your skin goes white and fragile. We had to change it at least twice a day, and let the skin breathe for longish periods. It took just over a week of this, and then suddenly the thing gave up the ghost.