I think the best evidence against it by using reverse psychology like the Prophet saaws.

Remember the words of the Prophet (peace and blessings of Allaah be upon him) to the one who came to him and asked him for permission to commit zina: “Would you like it for your mother?” He said: “No, by Allaah, may Allaah cause me to be sacrificed for you.” He said: “And people do not like it for their mothers.” He said: “Would you like it for your daughter?” He said: “No, by Allaah, O Messenger of Allaah, may Allaah cause me to be sacrificed for you.” He said: “And people do not like it for their daughters.” He said: “Would you like it for your sister?” He said: “No, by Allaah, may Allaah cause me to be sacrificed for you.” He said: “And people do not like it for their sisters.” He said, “Would you like it for your paternal aunt?” He said: “No, by Allaah, may Allaah cause me to be sacrificed for you.” He said: “And people do not like it for their paternal aunts.” He said, “Would you like it for your maternal aunt?” He said: “No, by Allaah, may Allaah cause me to be sacrificed for you.” He said: “And people do not like it for their maternal aunts.”
Narrated by Ahmad, 22265; classed as saheeh by Shaykh Shu’ayb al-Arnaa’oot in Tahqeeq al-Musnad.

So in this case if someone wants to chat ask them if they would let their sister, mother, aunt, etc chat also with a nonmahram.

Also
It was narrated from Abu Hurayrah that the Prophet (peace and blessings of Allaah be upon him) said: “Allaah has decreed for every son of Adam his share of zina, which he will inevitably commit. The zina of the eyes is looking, the zina of the tongue is speaking, one may wish and desire, and the private parts confirm that or deny it.”

Narrated by al-Bukhaari, 5889; Muslim, 2657.