No body is prosecuting them for believing in Mirza Ghulam Ahmed, they get prosecuted for portraying themselves as muslims, It was in practice that these Ahmadies, use to portray themselves as cult of Islam and make relationship with Muslim familes and get their sons married with the daughters of the muslim families...
You have right not to believe them & you also have right to teach people about what is a right path (according to your beliefs), but you cannot persecute another group for their beliefs. If they think they're muslims its their right. Why should they be persecuted for that?