agree = 0;
<div>
<form name="enableform">
<textarea name="formtextarea1" rows="5" cols="47">
회원약관
</textarea>
<br>
<table align="center" cellpadding="0" cellspacing="0" width="182">
<tr>
<td width="172">
<p align="left"><input type='radio' name='enable' value='agree' onClick="agree=1; document.enableform.input.focus();">약관에 동의함<br>
</td>
</tr>
<tr>
<td width="172">
<p align="left"><input type=radio name='enable' value='disagree' onClick="agree=0; document.enableform.input.value='';">약관에 동의하지않음
</td></tr>
</table>
<p align="center">
주민등록번호 :
<input type="text" name="input" onFocus="if (!agree)this.blur();" onChange="if (!agree)this.value='';" size="14">- <input type="text" name="box2" onFocus="if (!agree)this.blur();" onChange="if (!agree)this.value='';" size="15">
<br><br>
<input type="submit" value=" 확인 ">
</p>
</form>
</div>