select FilterNonAsciiChars( mycol, '?' )
  from MyTable;

update MyTable
   set mycol = FilterNonAsciiChars( mycol )
 where mycol not regexp '[[:ascii:]]*';

CREATE FUNCTION FilterNonASCIIChars( in @sInput     long varchar,
                                     in @bad_marker char(1) default '' )
RETURNS long varchar
DETERMINISTIC
BEGIN
    declare @pattern   long varchar;
    declare @iNotValid integer;
    declare @sOutput   long varchar;
    declare @char      char(1);

    set @pattern   = '%[^' || ' !"#$%&''()*+,./0-9:;<=>?@A-Z[\^_`a-z{|}~-' || ']%';
    set @sOutput   = '';
    set @iNotValid = patindex( @pattern, @sInput );

    while @iNotValid > 0 loop
        // pick up everything to the left of the non numeric
        set @char = substr( @sInput, @iNotValid, 1 );
        if @char = ']' then
            -- patindex cannot not catch the ']' character,
            -- so we need to explicitly ignore it
            set @sOutput = @sOutput || left( @sInput, @iNotValid );
            set @sInput = substr( @sInput, @iNotValid+1 );
        else
            -- allow all chars prior to the one we found
            set @sOutput = @sOutput || left( @sInput, @iNotValid - 1 );

            // strip off the invalid character and any similar chars in the string
            set @sInput = replace( substr( @sInput, @iNotValid+1 ), @char, @bad_marker );
        end if;

        // find the next invalid character
        set @iNotValid = patindex( @pattern, @sInput );
    end loop;

    // return our string plus any remaining chars
    return @sOutput || @sInput;
END;