We know we don't really need a specific canon. Ecumenists are in communion with heretics already condemned and that does not need a new canon. But we HAVE an ancient canon against ecumenism.
Carthage 418/419 AD, Canon 132 (Rudder) or 123 (Schaff) wrote:If in the mother cathedrals a bishop should have been negligent against the heretics, let a meeting be held of the neighbouring diligent bishops, and let his negligence be pointed out to him, so that he can have no excuse. But if within six months after this meeting, if an execution was in his own province, and he had taken no care to convert them to Catholic unity, no one shall communicate with him till he does his duty. But if no executor shall have come to the places, then the fault shall not be laid to the bishop.
Continuing the next Canon:
If it shall be proven that any bishop has lied concerning the communion of those [who had been Donatists], and had said that they had communicated when he knew it was an established fact that they had not done so, let him lose his bishopric.
These Holy Canons were endorsed by Canon 2 of Trullo and are thus oecumenically accepted. Even if the exact application of discipline can be creatively restricted to Donatists, this condemns the anti-proselytism of both ecumenist and Sergianist hierarchs along with the false claims of heretics being in communion with the Church when they have not returned.
You may be thinking: “Wait--it says only if the executors of the state come to his aid,” but that makes our case stronger. They did not invoke the state to assist in suppressing heresy, which would have been the case under the Empire, but in fact both Sergianists and ecumenists successfully petitioned the state to do the exact opposite of what this canon demands of them.
Why aren't we roasting anyone over a slow fire with this? Surely we're not going to let them get away with just insisting that since it was not an anathema, we can ignore it again.