symbols work if you believe in them. if you feel a pentacle is a symbol of the elements and can protect you, then it will. if you believe a crucifix is just a cross on the wall, then it won't. many pagans don't believe in Christian concepts [minus Christian Wiccans and the like] so carving angels on a board might not yield any protective results.
in regards to you views that an angel symbol will only call upon angels, doesn't work that way. a Ouija Board is more like a spiritual magnet, the person using the board pools energy into it, causing spirits, demons, and sometimes astral beings to be drawn towards the board. the spirit [or whatever] then channels through the user [or both if it's multiple people] to move the planchette and speak. casting a protective circle, and asking to speak to either a specific being or only a 'friendly spirit' are ways to protect yourself, but nothings ever that simple with a Ouija Board. they're not evil, they're a tool, but a powerful tool you need to be cautious around as a result. cleanse the area, people, and sometimes the board [depending on who you contacted and how many uses since the last cleansing] after every use to get rid of any lingering spirits/energy. if you feel it return, then you can cast a protection spell calling on angels, or hang an angel in the room to protect it. [if you believe in angels that is]