I do not believe this is possible so un-install ALL packages at once. However, as you already indicated you can un-install a package, but you can also tell it to un install its dependencies doing the following:
To get all packages from all projects in the solution use Get-Package. To get all packages from a specific project use Get-Package -ProjectName "YourProjectName".
Remove all packages from all projects in the solution
Be careful: This will uninstall ALL packages in the solution. If -Force parameter is used, packages are removed even if dependencies exist.
Using the -Force parameter in my case left project file modifications and references to some binaries that should have been removed when normally uninstalling the packages.
Here is a naive method to uninstall all packages from specific projects without using the -Force parameter. Effectively it tries to uninstall the packages over and over again until there are no packages left, so you will see some errors mentioning dependent packages (if you have them) but they will turn up less and less as the leaf packages get removed each iteration.
Also worth mentioning I've only tested the following PowerShell snippets in the PackageManager console. ("Tools > NuGet Package Manager > Package Manager Console")
Uninstall all the packages from all the projects in a solution
Note that if you have another issue apart from dependent packages preventing an uninstall of the package this snippet will run forever until you manually stop it.