You can spy on the prototype of object before the test starts. So it will not have any impact of your other tests.
var function A {
$.proxy(this, 'methodName');
}
a.prototype.methodName = function() {
console.log('test');
}
describe('…', function() {
var a;
before(function() {
jasmine.spyOn(a.prototype, 'methodName');
a = new A();
});
it('should…', function() {
});
});