Calico uses Linux's connection tracking ('conntrack') as an important optimization to its processing. It generally means that Calico only needs to check its policies for the first packet in an allowed flow—between a pair of IP addresses and ports—and then conntrack automatically allows further packets in the same flow, without Calico rechecking every packet.
This can, however, make it look like a Calico policy is not working as it should, if policy is changed to disallow a flow that was previously allowed. If packets were recently exchanged on the previously allowed flow, and so there is conntrack state for that flow that has not yet expired, that conntrack state will allow further packets between the same IP addresses and ports, even after the Calico policy has been changed.
Per Calico's current implementation, there are two workarounds for this:
Somehow ensure that no further packets flow between the relevant IP addresses and ports until the conntrack state has expired (typically about a minute).
Use the 'conntrack' tool to delete the relevant conntrack state; for example
conntrack -D -p tcp --orig-port-dst 80.
Then you should observe that the new Calico policy is enforced for new packets.