The prohibition of idolatry is a bit subtle. On one hand like my learning partner David Bronson explained to me that one can not be guilty of the death penalty unless he worships a physical object.
But even not of the side of legal guilt, clearly there is guilt if one worships in the abstract.
You can see this in the Rambam in the 13 principles of faith [and also in Mishne Torah] that idolatry is worship of anything in heaven or earth, any of the four elements, any angels etc. That is anything other that the First Cause.
But what is worship exactly? Well there are the regular four services [sacrifice, bowing, burning pouring] that one is guilty, but then there is included the specific type of worship that is done for that idolatry even if it is not one of the four services.
And Rav Shach explains the basic essence of idolatry is the fact that someone believes that idol can save. That is how he explains the argument between R Yochanan and Reish Lakish about an idol that fell apart by itself. So people would say, "If it can not help itself, how can it help me?"
We see a lot of idolatry in the religious world. And there is in particular a commandment to get rid of it whether in Israel or outside of Israel. [Outside of Israel one is still required to get rid of idolatry but not required to pursue it.] Clearly this was the reason the Gra signed the letter of excommunication, and it seems to me to be a good idea to take that letter seriously,--even to the degree of considering it valid according to the letter of the law.
Plus there is a further result: Idolatry is not just forbidden to do, but any objects that are set aside for use of idolatry are also forbidden.