That looks like what is called a security Allen bolt. It is designed to prevent someone without the special security Allen wrench to turn the bolt. Your easiest solution is to buy a set of security Allen wrenches. Unfortunately, you will likely have to buy both Metric and SAE since you do not know which the bolt is. Alternatively, you can use a Dremel with some fine bits and grind down the center post to allow the use of a normal Allen Wrench. I find the use of a security bolt on a motorcycle to be quite unusual.