You can set the default for new users in your:
Admin CP -> Home -> Options -> User Registrations -> Default Registration Values -> Show online status
You can update the preferences of existing users by running this query on your database:
Code:
UPDATE xf_user
SET visible = 1
You can remove the option to change this setting by removing some code from the templates:
Admin CP -> Appearance -> Templates -> account_privacy
Code:
<dl class="ctrlUnit surplusLabel">
<dt><label>{xen:phrase activity_display}:</label></dt>
<dd>
<ul>
<li><label for="ctrl_visible"><input type="checkbox" name="visible" value="1" id="ctrl_visible" class="OptOut" autofocus="autofocus" {xen:checked "{$visitor.visible}"} /> {xen:phrase show_your_online_status}</label> <p class="hint">{xen:phrase this_will_allow_other_people_to_see_what_page_you_currently_viewing}</p></li>
</ul>
</dd>
</dl>
Admin CP -> Appearance -> Templates -> account_preferences
Code:
<dl class="ctrlUnit">
<dt></dt>
<dd><ul><li><label><input type="checkbox" name="visible" value="1" {xen:checked $visitor.visible} /> {xen:phrase show_your_online_status}</label> <p class="hint">{xen:phrase this_will_allow_other_people_to_see_what_page_you_currently_viewing}</p></li></ul></dd>
</dl>
Admin CP -> Appearance -> Templates -> navigation_visitor_tab
Code:
<ul class="col1 blockLinksList">
<li>
<form action="{xen:link account/toggle-visibility}" method="post" class="AutoValidator visibilityForm">
<label><input type="checkbox" name="visible" value="1" class="SubmitOnChange" {xen:checked $visitor.visible} />
{xen:phrase show_online_status}</label>
<input type="hidden" name="_xfToken" value="{$visitor.csrf_token_page}" />
</form>
</li>
</ul>
Was a pleasure working with you 
