The database table AstorBase : GridMultiLang has a column CharsToBeExcluded that you can edit as you wish to control which letters do not get used.
The grid refs are generally set by the architect or engineers so we have to follow suit and since it can vary from one project to the next, editing this table is not very efficient.
The grid dialog could have a checkbox to either use all characters or the prefered ones as per above database.
...but better still the dialog could have a field to enter any characters to omit so it is easy to adjust "on the fly".