First update the security stamp of that user:
await userManager.UpdateSecurityStampAsync(user)
Then that user won't be noticed the changes until the arrival of the SecurityStampValidationInterval. So set it to Zero for the immediate logout:
services.AddIdentity<User, Role>(identityOptions =>
{
   // enables immediate logout, after updating the user's stat.
   identityOptions.SecurityStampValidationInterval = TimeSpan.Zero;
}
Update: For ASP.NET Core Identity 2.x, 3.x, 5.x
services.Configure<SecurityStampValidatorOptions>(options =>
{
    // enables immediate logout, after updating the user's stat.
    options.ValidationInterval = TimeSpan.Zero;   
});