Mine

using System;
using System.Collections.Generic;
using System.Windows.Forms;

namespace KeyPressTest
{
	static class Program
	{
		[STAThread]
		static void Main ()
		{
			Application.EnableVisualStyles ();
			Application.SetCompatibleTextRenderingDefault (false);
			Application.Run (new TestForm ());
		}

		public class TestForm : Form
		{
			private System.Windows.Forms.TextBox textBox1;
			private System.Windows.Forms.Label label1;
			private CheckBox cb;
			
			public TestForm ()
			{
				cb = new CheckBox ();
				cb.Text = "Ignore Enter";
				Controls.Add (cb);
				this.textBox1 = new System.Windows.Forms.TextBox ();
				this.label1 = new System.Windows.Forms.Label ();
				this.SuspendLayout ();
				this.textBox1.Location = new System.Drawing.Point (16, 68);
				this.textBox1.Multiline = true;
				this.textBox1.Size = new System.Drawing.Size (332, 84);
				this.textBox1.TabIndex = 0;
				this.textBox1.KeyPress += new System.Windows.Forms.KeyPressEventHandler (this.textBox1_KeyPress);
				this.label1.Location = new System.Drawing.Point (16, 16);
				this.label1.Size = new System.Drawing.Size (332, 48);
				this.label1.TabIndex = 1;
				this.label1.Text = "To reproduce the problem, type \"12345\" and the hit the Return (Enter) key. The character \"3\" and the Return key should be ignored. Instead, only the character 3 is ingored.";
				this.ClientSize = new System.Drawing.Size (364, 164);
				this.Controls.Add (this.label1);
				this.Controls.Add (this.textBox1);
				this.Text = "Form1";
				this.ResumeLayout (false);
				this.PerformLayout ();
			}

			private void textBox1_KeyPress (object sender, KeyPressEventArgs e)
			{
				if (cb.Checked)
					if (e.KeyChar == '3' || e.KeyChar == '\r')
						e.Handled = true;
			}
		}

	}
}