How to disable wrapping on some text part in RTFTextBox?

Why exactly are you using a RTF textbox? Isn't a DataGridView way more efficient and better looking?

You could make one with 2 columns, One for the Move number and the actual details. That way people can sort the moves ascending or descending and is counting all the moves as easy as checking the count number inside the DataGrid.

I know this is not an awnser to your question.. just a suggestion :)

DataGridView dataGridView = new DataGridView();
dataGridView.Columns.Add("Id", "#");
dataGridView.Columns.Add("Details", "Details");

// When adding a move;
dataGridView.Rows.Add((dataGridView.Count + 1), "* Your string with details");

DataGridView is no real option here, as the text is not only presented as a table, but also as flowing text, i.e. in variants:

1. e2-e4 e7e5
(Also possible is 1. d2-d4)

The example is trivial and short, but you get the point... I don't want breaks within "1. d2-d4", or the team will kill me :-)