The only names of Allah that you aren't allowed to use are ar-Rahmaan, al-Quddus, and al-Mutakkabbir and a few others. For the rest of the names, you are allowed to use it if you remove the "al" or "ar" in front of it. Therefore you shouldn't use ar-Raheem or al-Raheem, but Raheem itself is acceptable. The reason is that the "al" or "ar" at the front makes the name specific, thus ar-Raheem means "The Most Merciful". But when you remove the "al" or "ar", the meaning becomes a general Arabic word, thus "merciful", which is a good meaning.
Those who say that the attributes of Allah cannot be used as names are speaking without knowledge. The name of Ali, one of the most common names in Islam and the name of imam Ali bin Abi Talib, cousin of Prophet Muhammad, peace and blessings up him and his family and companions, is also derived from one of the 99 name of Allah: Al-Ali (The High One). Thus you cannot use Al-Ali as a name, but when you remove the "al" from the beginning, you get the acceptable name Ali, which means high, exalted, sublime.