Hacky way to prevent new message handlers from firing while in conversation (_on_edit, _on_read, and _check_custom are left as an exercise for the reader)

hack.py

import functools

from telethon import events
from telethon.tl.custom import Conversation


HANDLED_BY_CONV_ATTR = '_handled_by_conv'


@client.on(events.NewMessage)
async def on_msg(event):
  if getattr(event.original_update, HANDLED_BY_CONV_ATTR, False):
    raise events.StopPropagation


def attr_setter_wrapper(func):
  @functools.wraps(func)
  def wrapper(self, response):
    func(self, response)
    if self._incoming and self._incoming[-1] is response.message:
      setattr(response.original_update, HANDLED_BY_CONV_ATTR, True)
  return wrapper


Conversation._on_new_message = attr_setter_wrapper(Conversation._on_new_message)