Datacenter IP is the first thing most teams block on, and on its own it is a terrible reason to refuse anyone.
It tells you the request did not come from a phone or home broadband — it came from a rented server. That is genuinely useful information. It is just not a verdict, because the population it catches is mostly VPN users, and VPN users are disproportionately the security-conscious, privacy-aware, technically-literate people you would quite like as customers.
Same for disposable email domains: often it is someone protecting their inbox from a product they have not decided to trust yet. Same for a freshly registered domain: sometimes it is a founder who launched last week and is signing up for tools.
Each of those in isolation is a reason to look closer. None is a reason to refuse.
Where it becomes unambiguous is combination. A disposable address, from a datacenter address, on a domain registered yesterday, as the fortieth signup from that subnet this hour — nobody is arguing that is a customer having a bad day.
The general shape: single strong signals are how you refuse real people who never tell you. Combinations are how you catch the thing you were actually looking for. And blocking on any one field is almost always a decision someone made because it was the easiest thing to query, not because it was the most accurate.
Which single signal does your stack block on that you suspect you should not?