Limit characters in a text box added with javascript

I am using the following link & script to add a value to a text box. Could someone help me limit this so it can only be run once or limit the text box to not allow more than 7 characaters? Maxlength on the text box doesn't work. If I click the link twice it will enter the value twice right next to eachother.

<a href="##" value="happy" onclick="addme(this.value)">Add it</a>

<script>
    function addme(iii) {
    var txt=document.getElementById("theTxtBox").value;
    txt=txt + iii;
    document.getElementById("theTxtBox").value=txt;
    }
</script>

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