It is against the rules. Rules which you probably have not read. Depending on the case one could classify it as thread hijacking.
I have read "the rules".
This version:
https://bitcointalk.org/index.php?topic=994018.0There is nothing similar to "thread hijacking" and talking about bitcoin accepting digital services "in other's thread".
*Facepalm* Those are the rules for Russian board, which also by the way should be similar to the "Unofficial" rules of bitcointalk:
https://bitcointalk.org/index.php?topic=703657.0