Making linear mode the only mode

ACP > Templates > Showthread Templates > showthread > find the following and remove

Code:
<a href="showthread.php?mode=threaded&amp;tid={$tid}&amp;pid={$pid}#pid{$pid}">{$lang->threaded}</a> |

This'll remove the link from Showthread Pages.

ACP > Templates > User Control Panel Templates > usercp_options > find the following and remove

Code:
<tr>

<td colspan="2"><span class="smalltext">{$lang->thread_mode}</span></td>

</tr>

<tr>

<td colspan="2"><select name="threadmode"><option value="">{$lang->use_default}</option><option value="linear" {$threadview['linear']}>{$lang->linear}</option><option value="threaded" {$threadview['threaded']}>{$lang->threaded}</option></select></td>

</tr>

Now to make all Users with Linear Mode, Run the following query in phpmyadmin

Code:
UPDATE mybb_users SET threadmode = 'linear'