c# function returning letters from a string that aren't in another

Let's say I have two strings: main_word: 'abcdefa' and check_word: 'abcd'.

I know that the letters from check_word are all in the main_word.

I have to write a function that would return me the rest of the main_word after 'using all the letters to form the check_word. In the above example the function would return the string efa.

Here's my code:

private static string getResidue(string, main_word, string check_word)
{
    string result = "";
    bool isFound;

    foreach (char c in main_word)
    {
        isFound = false;
        for(int i = 0; i < check_word.Length; ++i)
        {
            if (c == check_word[i])
            {
                //check_word[i] = 'x'; mark as used (this doesn't work) 
                isFound = true;
                break;
            }
        }
        if (!isFound) result += c;
    }

    return result;
}

The problem is that this version doesn't support duplicates of letters. I commented the version that would solve my problem, but unfortunately, the c# doesn't allow that line because the property of indexer cannot be assigned to - it is read only. Any ideas how to make this function work as intended?