Is there any build-in mechanism to retain neutrality in the verification process for bitcoin miners? I am thinking of a situation where I offer miners a fee not to include transactions from a certain wallet in their blocks to slow down/ fully prevent settlement of certain trades (not that I have a specific use-case in mind, it is more something I consider an interesting game in terms of setting the right incentives for miners).
No, there is no such mechanism. Bitcoin leaves each miner entirely free to decide how to include valid transactions in the blocks they mine however they choose.
With less than 51% of mining power conspiring, at worst they could make transactions take about twice as long. With more than 51% of mining power conspiring, bitcoin loses a lot of its desirable properties.
A lot of things prevent this in practice though. For one thing, if there was evidence that 51% of mining power was actually conspiring to censor or blackmail, the users could get together and change the proof of work algorithm, rendering everyone's mining hardware useless. This is kind of the nuclear option for the users because it will also cost honest miners a lot of money and without honest miners, bitcoin is not useful or secure. Miners know that users have this nuclear option, so they're very unlikely to conspire in such an obvious way.