Workaround:
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        this.radRichTextEditor1.IsSpellCheckingEnabled = true;

        this.radRichTextEditor1.Insert("SOooome wrrrrong wooooooooords.");

        this.radRichTextEditor1.CommandExecuted += radRichTextEditor1_CommandExecuted;
    }

    bool shouldProcess = true;
    private void radRichTextEditor1_CommandExecuted(object sender, Telerik.WinForms.Documents.RichTextBoxCommands.CommandExecutedEventArgs e)
    {
        if (e.Command is DeleteCommand)
        {
            if (shouldProcess)
            {
                shouldProcess = false;
                RichTextEditorInputBehavior behavior = this.radRichTextEditor1.InputHandler;
                behavior.ProcessKeyDown(new KeyEventArgs(Keys.Back));    
            }

            shouldProcess = true;
        }
    }
}