Welcome to the forums!
Generally, the way I try to deal with people like that is to first have a peaceful, civilized conversation. Stay calm, don’t be aggressive and explain how that’s a problem for everyone. If it happens on a server where I’m well established, I also accompany it with an offer to help by donating some of the materials they might need for the inconvenience of moving or redesigning that part of their base.
If that doesn’t work out, your options are very limited, but not as limited as they were before the new rules were introduced. I can’t say whether the rules would cover this particular case or not, but that’s something you could try if the negotiation fails.
My final bit of advice would be to avoid using the threat of new rules when you try to negotiate. Nothing puts other people’s back up more than being threatened, regardless of whether you’re right. If you do that, they’ll certainly refuse to unblock the spawn and then your only option is to report them. If the report doesn’t result in an intervention, then you’re stuck with someone determined to keep that area blocked on that server as long as you play there.
I hope that helps. Good luck!