When you're dealing with bribery, you can't prevent blackmailing/fake reports at all.

What you can do, though, is to provide HTTP, HTTPS and TOR access to your webservice to allow everyone to submit stuff at his/her own security choice.

If possible, change your IP address(es) and domain names on random intervals; you can use free domains like .de.vu and friends to have reliable "entry points" (an example is canna.cu.to).

If you have an android app anyway, try building a "distributed" service like BitTorrent or Bitcoin for information exchange.