Edit in JSFiddle

function () {
    // We'll need to add to this array all the accounts we want to check 
    var gaAccounts = ['UA-XXXXXXX-Y'];

    // The var that is gonna hold the block switch that will returning by the macro
    var blockGTM = false;

    // Let's check for the presence of the legacy tracker object  
    if (window._gat) {
        // Looping thru all the available objects to find their assigned UAs and match it against our defined UAs array 
        for (var i = 0; i < _gat._getTrackers().length; i++) {
            for (var j = 0; j < gaAccounts.length; j++)
                if (gaAccounts[j] == _gat._getTrackers()[i]._getAccount()) blockGTM = true;
        }
    }

    // Let's check for the presence of the legacy tracker object  
    if (window.ga) {
        // Looping thru all the available objects to find their assigned UAs and match it against our defined UAs array 
        for (var i = 0; i < ga.getAll().length; i++) {
            for (var j = 0; j < gaAccounts.length; j++)
                if (gaAccounts[j] == ga.getAll()[i].get('trackingId')) blockGTM = true;
        }
    }
    return blockGTM;
}