not sure if the files system is similar to a Tomtom, but if it is you should be able to navigate through the operating files on your gps and manually delete unwanted poi.
** recommend you do a complete backup of you gps before removing files just incase you remove something of importance**
Also I dont think there is a limit on the amount of poi other than physical memory size, although you might find to many poi may slow you gps operating system down, taking longer to display mapping screens.
The S series has a POI limit of 12000 which is a guide....... more than this is will impact on the speed and performance of the SATNAV. After your initial play with POI's be more selective with which files you upload. If you have a seperate camera detector ie road angel or snooper then there is no reason to add mobile speed cameras which I believe there are 14000 in the UK, or you can just filter and add the cameras for your area ie England, Scotland or Wales, which will reduce your number of POI's.