Function to prevent users from trashing comments

First, a note on you code: if you don’t encapsulate the code of a conditional with curly brackets: if($something){ encapsulated_actions(); }
it will only execute the next line.

So, you should use:

if( 'post' != get_post_type() ) // if different of desired type, do nothing and return default
    return $actions; 

// do your thing and return modified var
unset ($actions['spam'], $actions['trash'] ); 
return $actions;

Or:

if( 'post' == get_post_type())
{
    unset ($actions['spam']); 
    unset ($actions['trash']); 
    /* The following is best use */
    // unset ($actions['spam'], $actions['trash'] ); 
}
// returns modified or unmodified var depending on previous check
return $actions;

We need to target four places to block/hide the Trashing of comments. Here, I’m checking for administrative role. To check for specific users, use get_current_user_id().

The function names indicate the ID of the original Question here in WordPress Answers, e.g., wpse_92313 is: Comments screen in backend, how to disable Quick Edit | Edit | History | Spam | for non admins

1) Quick edit action row

add_filter( 'comment_row_actions', 'comments_row_wpse_92313', 15, 2 );   
function comments_row_wpse_92313( $actions, $comment )
{
    /* Type of the parent post */
    // $parent_type = get_post_type( $comment->comment_post_ID ) );

    if( !current_user_can( 'delete_plugins' ) )
        unset($actions['trash'] );

    return $actions;
}

2) The Views row ( All | Pending | ... | Trash )

add_filter( 'views_edit-comments', 'wpse_30331_custom_view_count', 10, 1);   
function wpse_30331_custom_view_count( $views ) 
{
    if( !current_user_can( 'delete_plugins' ) )
        unset( $views['trash'] );

    return $views;
}

3) Bulk actions

I’ve just discovered the filter hook 'bulk_actions-' . $this->screen->id

add_filter( 'bulk_actions-edit-comments', 'remove_bulk_trash_wpse_92534' );   
function remove_bulk_trash_wpse_92534( $actions )
{
    if( !current_user_can( 'delete_plugins' ) )
        unset( $actions['trash'] );

    return $actions;
}

4) Blocking direct access to trash screen

Redirects example.com/wp-admin/edit-comments.php?comment_status=trash to the main Comments screen.

add_action( 'admin_head-edit-comments.php', 'wpse_74488_block_trash_access' );   
function wpse_74488_block_trash_access()
{
    // Don't run the function for Admins
    if( current_user_can( 'delete_plugins' ) )
        return;

    if( isset( $_GET['comment_status'] )  && 'trash' == $_GET['comment_status'] ) 
    {
        wp_redirect( admin_url( 'edit-comments.php' ) ); 
        exit;
    }
}