With sites that you dont want people to be able to register with, this is my solution. It involves taking the login module off the menu and making a separate page that only you would know. For the page you'll need the code below. Make sure you have set the input format to 'full html'. Basically this is just a block of code chopped off another page which needs a slight modification to work properly. This is the block of code:

<div class="column1-unit">
                                    <form action="/?q=user/login&destination=node/4"  accept-charset="UTF-8" method="post" id="user-login">
<div><div class="form-item">
 <label for="edit-name">Username: <span class="form-required" title="This field is required.">*</span></label>

 <input type="text" maxlength="60" name="name" id="edit-name"  size="60" value="" tabindex="1" class="form-text required" />
 <div class="description">Enter your Angus's Website username.</div>
</div>
<div class="form-item">
 <label for="edit-pass">Password: <span class="form-required" title="This field is required.">*</span></label>
 <input type="password" name="pass" id="edit-pass"  size="60"  tabindex="2" class="form-text required" />
 <div class="description">Enter the password that accompanies your username.</div>

</div>
<input type="hidden" name="form_id" id="edit-user-login" value="user_login"  />
<input type="submit" name="op" id="edit-submit" value="Log in"  tabindex="3" class="form-submit" />

</div></form>

The two parts that need modifying are:
'

/?q=user/login&destination=node/4

'
at the top of the block, the destination= must be the same as the page that the login will be placed on otherwise I've found it doesnt work.
The other bit is the part that says 'Enter your Angus's Website username.' - this would have to be changed for your particular website name.