It isn't possible, but you can choose which subs where you want to subscribe.
There will be a moderated list of subs (maintained by the main developer), but users will be free to try to find an join even the most controversial ones.
You can instant accept any bitcoin value that is lower to the investment to do the 51% attack or similar.
Example: no one is going to make a 51% attack to steal only 10 bitcoin from a vending machine.
Simply put a limit, or just say to the user that any amount of bitcoin over X will need Y time to get verified.
Anything lower that X amount will be instantly processed.