Hi, well there isn't an interface within the product to easily remove it but lets not let that stop us.
Find file Login.cshtml that is under E-Forms\View\Account and change it so the option is unchecked and hidden.
<span class="checkbox" style="margin-left:20px;display:none;">
<label for="RememberMe">
@Html.CheckBoxFor(m => m.RememberMe)
@Resources.STR_LOGIN_REMEMBER_ME
</label>
</span>
So I added a display:none; to the parent element and I removed 'new { @checked = "checked" }' so it defaults to unchecked.
Just be careful on upgrades because the change will be overwritten and make sure to keep a copy of the original in case you want to go back or run into trouble.