You don’t clarify if this is a temporary beam during construction, or one that will stay permanently.
If it’s temporary, and the neighbor is frum, then you should just let it go, even though what they did was not so nice. Also keep in mind that they may not even realize it’s on your property or the contractor may have done it without them knowing.
If however it’s permanent, then you need to take action. I suggest consulting your ruv about din torah vs. 311. While it may not be assur to call 311, assuming you live al pi tora, then you should consult a rav.