I've not worked with mod rewrite so I could easily be wrong, but it's my understanding that the mod rewrite would be able to change a request to remove the query string, but unless you have specific pages created in the mod rewrite for each possible value of the "num" parameter, then the page won't "know" what the parameter should be and therefore won't be able to display the proper data on the page. That would stop the robot from getting to your content, but also the humans that need it too. ;-)
Another option could be to not use autoincrement for the ID of the item to display, and generate the ID in some other fashion, like a text unique id, perhaps using uniqid
which will create a 13 character long ID, 23 characters long with the more_entropy parameter set to true.
This would require changes to your database, and making sure to generate the ID column value each time a record is inserted into the database.
However, many robots will follow links through out the site to find all of the available data items rather than simply incrementing query string parameter values. If the robot(s) coming to your pages spider rather than increment, then the unique id won't help in that regard, as if there is some way to get to the page, the spider will find it. You'd be better off limiting visits by IP over a certain timeframe.
In addition to limiting the number of requests, you could also "ban" IP addresses where if you find that they are running through your site, then add the IP to a "bad id" table in your DB and then spit back a 404 or some other HTTP error when a request comes through from a bad IP. The problem with this approach is that with some internet connections IP addresses change all the time, and therefore an outright ban could be stopping legitimate traffic. That's why I initially suggested the sleep response which will merely slow down the response to the bad requests. You could even take a progressive approach where if the request come in at over a certain rate per minute/hour/day, then you increase the delay on the response.