if (cbOdmazatPo.Text == ";")
{
    DeleteToChar(selectionStart, ';');
}

...

void DeleteToChar(int selectionStart, char chr)
{
    var lineIndex = richTextBox1.GetLineFromCharIndex(selectionStart);
    var start = richTextBox1.GetFirstCharIndexFromLine(lineIndex);
    var charIndex = richTextBox1.Text.IndexOf(chr, start);
    lineIndex = richTextBox1.GetLineFromCharIndex(charIndex);
    var end = richTextBox1.GetFirstCharIndexFromLine(lineIndex);
    richTextBox1.Text = richTextBox1.Text.Remove(start, end - start);
}