Case 1: They require belief in their godliness to sustain themselves.
Method 1A: Convert their followers.
Method 1B: Genocide their followers.
Method 1C: Combine 1A and 1B.
Case 2: They do not require mortal belief in order to remain in existance.
Method 2A: Nuke 'em.
Method 2B: Several sniper teams.
Method 2C: Tank division
Method 2D: Bombing, artillery and missile strikes.
Method 2E: Judicious application of a railgun.
Method 1A: Convert their followers.
Method 1B: Genocide their followers.
Method 1C: Combine 1A and 1B.
Case 2: They do not require mortal belief in order to remain in existance.
Method 2A: Nuke 'em.
Method 2B: Several sniper teams.
Method 2C: Tank division
Method 2D: Bombing, artillery and missile strikes.
Method 2E: Judicious application of a railgun.