You'll have to edit:
templates/default/postlist.tmpl (remove the link to expanded view in the table at the top)
templates/default/showflat.tmpl (remove the link to showthreaded.php at the top and bottom of the file
templates/default/editdisplay.tmpl (remove the two select boxes)
Open showthreaded.php
Search for:
$activethread = "";
$PNumber = "";
Insert before:
header("Location: {$config['phpurl']}/showflat.php{$var_start}Cat{$var_eq}$Cat{$var_sep}Number{$var_eq}$Number{$var_sep}an{$var_eq}$an{$var_sep}page{$var_eq}$page{$var_sep}vc{$var_eq}1");
This will redirect all requests from showthreaded.php to showflat.php
Open changedisplay.php
Search for:
$timeformat = get_input("timeformat","post");
$profilehash = get_input("profilehash","post");
Insert below:
$view = "collapsed";
$display = "flat";
Run SQL query to set preferences off all users to flat and collapsed view:
Update w3t_Users SET U_Display = "flat", U_View="collapsed";