Can Dusk shut people off the chain? Say i was the government, will i be able to restrict people selling securities or restrict other abilities on the chain?

Really depends on what scope you are talking about. Exclude people from participating in the network? No, it's permissionless, so everyone can use it. When it comes to securities however, it's gonna be a different story if they are issued with the XSC standard for example. There the issuer has to make sure he is compliant with security laws and has therefore in-built utility that would allow him to exclude participants from specific countries for example or force freeze assets when someone stole your private key.

Basically think of it as an open and permissionless network like ethereum. Per se its functionality is open to everyone, however dApps/smart contracts on Dusk can further limit their target audience or what people can do when they interact with it.