Disable C# 6.0 Support in ReSharper

Click the project node in the Solution Explorer. Then look in the Property Grid (F4). You'll see a property named "C# Language Level". Set that to "Default" or your desired language level.

TO disable it at once across the solution rather than per project level, please add below in .sln.DotSettings

<s:String x:Key="/Default/CodeInspection/CSharpLanguageProject/LanguageLevel/@EntryValue">CSharp50</s:String>

it would disable resharper 6.0 features/give you error

EDIT

as per comments, in case you dont have the sln.DotSettings file at all, then you need to create one with below contents

<wpf:ResourceDictionary xml:space="preserve" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:s="clr-namespace:System;assembly=mscorlib" xmlns:ss="urn:shemas-jetbrains-com:settings-storage-xaml" xmlns:wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation"> 
    <s:String x:Key="/Default/CodeInspection/CSharpLanguageProject/LanguageLevel/@EntryValue">‌​CSharp50</s:String>
</wpf:ResourceDictionary>