void Main()
{
    var lines = File.ReadAllLines(@"d:\temp\input.txt");

    var re = new Regex(@":(?<type>Allocation|Deallocation).*Item\s*:(?<id>\d+)\s*$", RegexOptions.IgnoreCase);
    var matches = new Dictionary<int, ReferenceMatch>();
    foreach (var line in lines)
    {
        Match ma = re.Match(line);
        if (!ma.Success)
            continue;

        int id = int.Parse(ma.Groups["id"].Value);
        matches.TryGetValue(id, out ReferenceMatch refMatch);
        if (StringComparer.CurrentCultureIgnoreCase.Equals(ma.Groups["type"].Value, "Allocation"))
            refMatch.Allocated = true;
        else
            refMatch.Deallocated = true;
        matches[id] = refMatch;
    }

    using (var writer = new StreamWriter(@"d:\temp\output.txt"))
    {
        foreach (var line in lines)
        {
            Match ma = re.Match(line);
            if (!ma.Success)
            {
                writer.WriteLine(line);
                continue;
            }

            int id = int.Parse(ma.Groups["id"].Value);
            matches.TryGetValue(id, out ReferenceMatch refMatch);

            if (refMatch.Allocated && refMatch.Deallocated)
                continue;
            writer.WriteLine(line);
        }
    }

    matches.Dump();
}

public struct ReferenceMatch
{
    public bool Allocated;
    public bool Deallocated;
}