It's a paid network. https://siasky.net/ is free for the moment, but will be switching to a freemium model in the near future. As a portal, siasky will have the ability to ratelimit and block abusers.

If you "abuse" the network using your own node, you will have to pay for everything you upload and download, which means the people storing it won't really think of it as abuse, you're just another paying customer.

Cost. If this abuser was running his own Portal node to do so, he would simply be paying hosts to host his junk data. Hosts would be very happy to do so, as they set their own prices.

If this abuser was trying to use a public Skynet portal, the same abuse prevention measures apply as for any other data storage service out there. IP banning, filtering, etc

I hope it’s not the T-800[1].

[1]- https://en.m.wikipedia.org/wiki/Terminator_(character)