GitCode, a git-hosting website operated Chongqing Open-Source Co-Creation Technology Co Ltd and with technical support from CSDN and Huawei Cloud.
It is being reported that many users’ repository are being cloned and re-hosted on GitCode without explicit authorization.
There is also a thread on Ycombinator (archived link)
Forks are derivative works (quite obviously) so yes you can forbid them via license terms. Whether or not that’s still open source, take it up with OSI. I vaguely recall that at least once upon a time there was some project that required modification to the code to be published as separate patches and it was generally accepted to be open source don’t ask me which.