您只需将 ListBox HorizontalAlignment 设置为 Left,并将 VerticalAlignment 设置为 Top。这应该可以解决问题。
简单的例子:
主窗口.xaml 
<Window x:Class="ListBoxFitItemsPanel.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="400" Width="400">
    <ListBox ScrollViewer.HorizontalScrollBarVisibility="Disabled" ScrollViewer.VerticalScrollBarVisibility="Disabled" Background="Red" HorizontalAlignment="Left" VerticalAlignment="Top" >
        <ListBox.ItemsPanel>
            <ItemsPanelTemplate>
                <WrapPanel Background="AntiqueWhite" Margin="5" />
            </ItemsPanelTemplate>
        </ListBox.ItemsPanel>
        <Rectangle Width="100" Height="100" Fill="LightSlateGray" Stroke="Black" StrokeThickness="1" Margin="5" />
        <Rectangle Width="100" Height="100" Fill="LightSlateGray" Stroke="Black" StrokeThickness="1" Margin="5" />
        <Rectangle Width="100" Height="100" Fill="LightSlateGray" Stroke="Black" StrokeThickness="1" Margin="5" />
        <Rectangle Width="100" Height="100" Fill="LightSlateGray" Stroke="Black" StrokeThickness="1" Margin="5" />
        <Rectangle Width="100" Height="100" Fill="LightSlateGray" Stroke="Black" StrokeThickness="1" Margin="5" />
        <Rectangle Width="100" Height="100" Fill="LightSlateGray" Stroke="Black" StrokeThickness="1" Margin="5" />
        <Rectangle Width="100" Height="100" Fill="LightSlateGray" Stroke="Black" StrokeThickness="1" Margin="5" />
        <Rectangle Width="100" Height="100" Fill="LightSlateGray" Stroke="Black" StrokeThickness="1" Margin="5" />
        <Rectangle Width="100" Height="100" Fill="LightSlateGray" Stroke="Black" StrokeThickness="1" Margin="5" />
    </ListBox>
</Window>

编辑:它也适用于绑定场景:
主窗口.xaml
<Window x:Class="ListBoxFitItemsPanel.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:ListBoxFitItemsPanel"
        Title="MainWindow" Height="400" Width="400">
    <Window.Resources>
        <DataTemplate DataType="{x:Type local:Item}">
            <Rectangle Width="100" Height="100" Fill="LightSlateGray" Stroke="Black" StrokeThickness="1" Margin="5" />
        </DataTemplate>
    </Window.Resources>
    <ListBox ItemsSource="{Binding Items}" ScrollViewer.HorizontalScrollBarVisibility="Disabled" ScrollViewer.VerticalScrollBarVisibility="Disabled" Background="Red" HorizontalAlignment="Left" VerticalAlignment="Top" >
        <ListBox.ItemsPanel>
            <ItemsPanelTemplate>
                <WrapPanel Background="AntiqueWhite" Margin="5" />
            </ItemsPanelTemplate>
        </ListBox.ItemsPanel>
    </ListBox>
</Window>
主窗口.xaml.cs
using System.Collections.Generic;
using System.Windows;
namespace ListBoxFitItemsPanel
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            DataContext = this;
        }
        public IEnumerable<Item> Items
        {
            get
            {
                for (int i = 0; i < 9; i++)
                {
                    yield return new Item();
                }
            }
        }
    }
    public class Item { }
}