To reproduce: add a RadSpellChecker and a RadTextBoxControl. Set the RadSpellChecker.AutoSpellCheckControl property to the RadTextBoxControl.
Please refer to the attached screenshot.

Workaround:
Dim tbSpellChecker As TextBoxSpellChecker = Me.RadSpellChecker1.GetControlSpellChecker(GetType(RadTextBoxControl))
Dim fi As FieldInfo = GetType(TextBoxSpellChecker).GetField("punctuation", BindingFlags.NonPublic Or BindingFlags.Instance)
Dim chars As Char() = " .,;:!?-\\/|~!@#$%^&*()_+=".ToCharArray()
fi.SetValue(tbSpellChecker, chars)