<ImplicitUsings>disable</ImplicitUsings>
has to be added to the PropertyGroup in the csproj-file.
For example:
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFramework>net6.0</TargetFramework>
<Nullable>enable</Nullable>
<ImplicitUsings>disable</ImplicitUsings>
</PropertyGroup>
</Project>
If you are using a beta version of .NET 6, you will have to use <DisableImplicitNamespaceImports>true</DisableImplicitNamespaceImports>
.