Right. But can full nodes force miners to reject anything? If not then what full nodes think is valid or not doesn't effect the blockchain.
Full nodes can't force the miners to do anything. But full nodes (and their users) can reject anything the miners do.
In your scenario, the miners will be working away on their invalid blockchain, but the valid blockchain will stop growing. At that point all it takes in one miner doing CPU mining on an old Celeron to resume growing the valid blockchain. It may take several difficulty adjustment periods before the single miner can produce a block on schedule, but valid transactions will eventually resume. Meanwhile, the rogue miners will be unable to turn their BogusCoins into value.
One thing I have learned in the altcoin space is that value follows hashpower. A coin with high hashpower will have a higher value on average, and as a coins hashpower increases, so does the value. If the majority of Bitcoin miners start mining InvalidCoin fork, then that is the new Bitcoin. Could a node point out that there was an invalid transaction and that miners should stop on their chain and restart on that old block? Sure, but remember "One CPU one Vote", if the majority of miners just continue mining InvalidCoin, then the "CPU's" have spoken.
Forks happen all the time on altcoin chains and what it does is make it so the cartel is the only one who can mine and all other miners are out of luck. The minority miners can't convince anyone that they are actually the ones mining the real altcoin. Value follows hashpower.