It could literally just reject anything with asterisks.
It doesn't even need to do anything perfectly, it just needs to do it enough to produce hurdles for you. Like blowing through the number of attempts you realistically have remaining.
No, my idea (as you can see from my post) is: parse the SQL, then check the resulting structure (that is basically a whitelisting process) and then turn the structure into SQL again. The last part is crucial, because it means that you have turned a whitelisted structure into SQL. Or in other words: even if some evil person found a bug and was able to convince the sql parser that everything is fine even though it is not, they would not be able to leverage that, because you are not actually running their SQL.
Or to be more concrete: let's say the parser thinks that something is a comment, but the DMBS would actually run it as SQL - then that would be a problem if you jsut sanitize the SQL. But it's not a problem if you turn the SQL structure into SQL again, because your code to do that would just reject anything that it doesn't expect (and that definitely includes comments).