looks like this might be an option...
-webkit-touch-callout: none; /* iOS Safari */
-webkit-user-select: none; /* Safari */
-khtml-user-select: none; /* Konqueror HTML */
-moz-user-select: none; /* Old versions of Firefox */
-ms-user-select: none; /* Internet Explorer/Edge */
user-select: none; /* Chrome, Edge, Opera and Firefox */
but I think it is a bad idea to try to force a form control to do what you want here. I would make my own out of sprites... especially if people will be using this on a mobile device.