No idea about the printer.
For the iPad, you can only do it individually if the network is available. If it is, then go into Settings > Wi-Fi Networks, open the networks settings by tapping the blue arrow to the right, then choose Forget this Network from the top.
You can forget all networks by going to Settings > General > Reset, and Reset Network Settings. Be careful in this area and make sure you choose the right button. You can completely erase your iPad. There is a confirmation dialog, but I've been known to blow past these when I think I know what I'm doing.
This does not prevent Wi-Fi networks from appearing as a choice in the Wi-Fi settings, but it does stop you from automatically joining. You may still be asked if you want to join (at least the first time) if you have that option turned on.