You're using the wrong allen wrench. You either have metric bolts and you're using an SAE wrench, or vice-versa. This will round off the corners and ruin the bolt. Allen wrenches are cheaper than replacement bolts, so that's an easy choice.
If there isn't enough left to remove the bolt, you need a tool called an ez-out. It's designed to remove stripped bolts exactly like what you've got. Another option is to take it to a machine shop and pay tem to do it (heck they might even do it free, since it's about a 2 minute job).