I'm looking for a way to stop the currency from being automatically updated for EUR & USD, and instead set a fixed amount that will never change, is this possible?
Also Is there a way to apply a fixed amount for EUR and USD for each individual product?
If you want to set the currency rates manually, you don't need to keep the cron running. When you want to set the new rates just click on the import button, adjust the values (if needed) and save them.
Also there is no way to have the cron running but not keep the values. They are not saved anywhere temporary. If you want this you might need heavy customization. You should create a new table, run the cron to save the values in your table and then retrieve them from there when you want to change the rates.
You can disable the cron from System->Configuration->Currency Setup->Scheduled Import Settings