There are a lot of ways to ruin spots and many have already thrown them out already, but I'll add a couple:
Gum: when people leave their gum on a spot where it can get on everyone's gear.
Being too noisy beyond normal skateboarding sounds: loud music, mini party vibes, breaking shit, etc.
Overwaxing a spot: such bullshit. I know I don't skate super fast but I'll wax my set up on a street spot before the spot itself. Way to let people slip out to herniated disc in their spine.
Spilling beverages or food at the spot so it looks like Tony's first day at a busy Taco Bell. Fuck that! Eat before/after or do it wayyyy off to the side.
Blowing up the spot on social media unnecessarily with location tags.
Being rowdy/rude with local citizens of the area. This applies to spots in/near parks, on frequented public access ways, security even, etc.
Trash. Why can't people being a bag to put their trash in or use the trash can if one is nearby, for fuck's sake.
Spitting on the spot/runway/landing: unforgivable, especially if it gets on your shoes/board. Spitting off to the side away from where people congregate/use the spot is totally acceptable. Just because of Covid (
@ OP), people aren't allowed to spit because their body tells them it needs to get rid of this excess saliva/waste byproduct? That's a weird take, OP. Spitting is virtually harmless if you're not spitting on someone or into their general person-space.