Unfortunately, no. The symbols are there to stay--they bug me too--but there is a way around it, in a manner of speaking. Instead of automatic upgrading squadmates who have ammunition skills, you can manually go through that tree and pick that when they have a certain type of ammunition (Disruptor, Armor piercing, Incindiery, Cryo) equipped, then all squadmates including Shepard, receive the benefits of the ammunition. This results in a omni-tool tracing lining the gun colored like the ammunition type.
You only get half the effectiveness, and you can't turn it off. But hey, it gets rid of the symbols.
There's also the option of not using different ammunition types and instead using upgrade paths for modifications, weapon special abilities, and powers such as the grenades or biotics.