We were able to identify and block these public ISIS channels thanks to your reports. Thank you!
Here's an explanation of the blocking process from our FAQ:

All Telegram chats and group chats are private amongst their participants. We do not process any requests related to them. But sticker sets, channels, and bots on Telegram are publicly available. If you find sticker sets or bots on Telegram that you think are illegal, please ping us at abuse@telegram.org.

Our mission is to provide a secure means of communication that works everywhere on the planet. In order to do that in the places where it is most needed (and to continue distributing Telegram through the App Store and Google Play), we have to process legitimate requests to take down illegal public content (sticker sets, bots, and channels) within the app. For example, we can take down sticker sets that violate intellectual property rights or porn bots in countries where pornography is illegal.

User-uploaded stickers sets, channels, and bots by third-party developers are not part of the core Telegram UI. Whenever we receive a complaint at abuse@telegram.org regarding the legality of public content, we perform the necessary legal checks and take it down when deemed appropriate.

Please note that this does not apply to local restrictions on freedom of speech. For example, if criticizing the government is illegal in a country, Telegram won‘t be a part of such politically motivated censorship. This goes against our founders’ principles. While we do block terrorist (e.g. ISIS-related) bots and channels, we will not block anybody who peacefully expresses alternative opinions.

FAQ link: https://telegram.org/faq#q-there-39s-illegal-content-on-telegram-how-do-i-take-it-down