Block Devices From Connecting
Currently anyone on your network can connect to your Roku and do anything to it. There is no way to stop this. Patch this now.
Forum Discussion
Some users rely on this ability to connect to the Roku over the network in order to use certain features. (ie. web apps, casting, media servers).
If you wish to block all devices for your particular Roku usage, there are a couple different things you can do.
The first is to go to Settings/System/Advanced System Settings/Control by Mobile Devices/Network Access and select either [Limited] or [Disabled]. This will prevent most apps and software from communicating with/connecting to your Roku device.
Another thing to do is look to see if your router allows for "Client Isolation". Many routers offer this on the Guest Network while other routers will allow you to Enable this setting across the entire network. (most provide for at least enabling on guest networks to prevent them from accessing your network devices since they assume you trust others that are allowed to join the regular network ie. typical home use).
If you are aware of any particular app or 'threat' that you want to block from accessing your network, you could also try blocking it under other router settings that include ports, addresses, etc.
Another option is to introduce another Access point (or router) into the network setup just for the Roku devices and prevent other devices from joining it. (ie. either prevent other connections, or set this AP/router to only accept certain MAC addresses that you approve).
Sometimes the end user has to make some changes to their network/equipment when introducing devices to it that may behave in a manner they didn't expect.
