Очень просто:
<input type="radio" name="Name1" value="Value1" onmousedown="this.c=this.checked" onclick="if (this.c) { this.checked = false }" />
Очень просто:
<input type="radio" name="Name1" value="Value1" onmousedown="this.c=this.checked" onclick="if (this.c) { this.checked = false }" />