Limit wanted to High Contrast

At our Museum we use WordPress for visitors to learn more of the exhibits and local history. Today we had a problem with the “High Contrast” feature. Some visitor, obviously, clicked on the large letter ‘A’ (in ‘High Contrast) and kept on clicking until only two very large letters appeared on the screen – there was no room for anything else. We’ve never had the problem before so had no idea what had gone wrong. By accident, I hovered the mouse hovered over the letters and, for just a second, the words ‘high contrast’ appeared in a dialogue box.

I played around until I got a box that said ‘reset default’ and I managed to click on that before it disappeared – it was that quick.

Everything, then went back to normal but how can I limit the amount of enlargement that he button can do? Or is it best to remove the ‘High Contrast’ link?