I'm going to take this apart a little bit:
I am looking for a way to send files from a computer to a rpi without
the rpi being able to send anything to the pc and therefore possibly
get access the computer's network.
I hate to be a stickler, lol, but you need to clarify some things, because on a literal level this is not possible. ALL network communication requires a two way relationship. If you initiate a connection with another computer, the ONLY way for that connection to be established is for that computer to send back a response.
The rpi will be used as a
router/server that devices from the outside can connect to and access
If the other computers are supposed to access "certain data", how are they supposed to receive it if this is a "one-way" connection? Accessing data means sending a request, and then receiving a response. That's two-way communication.
That said, it is possible to use a firewall to allow a connection to be initiated from the outside (resulting in two way communication) but not from inside (meaning, the pi can't establish a connection, only reply to one). I presume that's what you are after.
If you are using some form of linux (eg. raspbian), the way to do that is via iptables. Here's a couple of tutorial links:
Basically, you want a set-up that rejects everything in the
OUTPUT chain except for things that have