you can disable registrations by going to the admin > site settings > advanced > security settings and set registration to 'none'

you could also go to the DesktopModules/authenticationservices/dnn/login.ascx and remove the following code

 <asp:Panel ID="pnlRegister" runat="Server" Visible="false">
    <table cellspacing="0" cellpadding="0" summary="User Design Table" border="0" width="350">
        <tr><td width="350" valign="bottom"><asp:label id="lblRegisterTitle" cssclass="Head" runat="server" resourcekey="RegisterTitle" /></td></tr>
        <tr><td width="350" valign="bottom"><asp:label id="lblRegisterHelp" cssclass="Normal" runat="server" /></td></tr>
        <tr><td width="350" height="10"></td></tr>
        <tr><td width="350" valign="top"><dnn:user id="ctlUser" runat="Server" /></td></tr>
        <tr><td width="350" height="10"></td></tr>
    </table>
    <dnn:commandbutton id="cmdCreateUser" runat="server"
        imageurl="~/images/save.gif"
        causesvalidation="True" />
</asp:Panel>