What would happen if miners refused halving In may 2020 the supply of bitcoin per block is supposed to be reduced by half https://www.bitcoinblockhalf.com/

What if after this phase the first miner or more than 51% of them decide to keep the current production rate ? Do their block will be refused like if they had produced a wrong block ?
What if after this phase the first miner or more than 51% of them decide to keep the current production rate ? Do their block will be refused like if they had produced a wrong block ?


Exactly. This is no different than if a miner (or a majority of miners) created blocks which paid out anything more that 12.5 BTC today: the network would simply ignore the new blocks, and the miners will have wasted their resources, working on an invalid block.

@JBaczuk's answer is technically correct that this would create a hard fork, but it ignores the bigger picture that even if a majority of miners make the switch, that doesn't mean users will follow. So while the miners could create a new, hardforked chain, this would be rather uninteresting, as there would be no users on it. As we've seen with past events (ie, blocksize debates, Segwit2x, etc), miners will follow users, not the other way around. 

If this were not the case, then users (and the network's rules) would be at the whim of the miners. This would create a broken system of incentives, as miners could just change the rules to benefit themselves (eg, changing the issuance of new coins).