A block with reward different from the current correct reward can not be valid. Doesn't matter if it's 1 Bitcoin more or even 1 satoshi less, if it's not precisely what it needs to be, such block is invalid. When a node tries to send invalid block to peers, they reject it and don't propagate it any further. This is why it's important to have a big decentralized network of nodes. If miners controlled all the nodes, like in a scenario that emerges from big block forks, then they could change the rules as they want, and users would only have 2 options - accept the rules or stop using Bitcoin at all.