Typecasts in LARPs can be a serious problem. In the future, I'd suggest avoiding getting into this situation in the first place - ask to play characters you're not usually cast as. This is difficult, so to the best of your ability, but the less you allow yourself to fall into a typecast, the less others will typecast you in turn.
That being said, it won't help you at this point. The above is a concern for future-you. The below is a concern for current-you. This might take a couple games to recover from, though, so don't be surprised if it doesn't change immediately. Typecasts are sticky, and it can be difficult to force people to see you differently, just as it is in real life.
I'm going to assume you've already done the obvious and told the other players directly that you're intending to play a nonviolent character. If you haven't done this, do this before anything else. Additionally, after games, discuss in whatever sort of postmortem you do what your expectations were for how others handled your character, and what you'd like to see in the future. If there were any nonviolent signals you broadcast that others missed, it might be worth pointing out to them so they don't miss them in the future.
Here's how you can specifically signal a change of character in-game:
Change your costuming. This is the easiest, so I list it first. If you're playing a mook, you almost certainly have a costume that doesn't change frequently - if at all. If you change it, that's a clear signal to the other players that you're not a mook. At the very least, it's a signal that you're something different than you are normally. This might be as simple a change as that of color, or type of weapon, but something needs to visually indicate that you're not who you normally are.
In short, the type of person you are is visible by your clothing.
For example, in a LARP I played with ~90 people, there were four people who were... distinctly different, and it needed to be apparent. Their costuming showed that they didn't belong with any group we'd ever seen before in-game, and so we knew immediately that a) they were of importance, b) they were scary, and c) we don't know anything about them.
That's what costuming broadcasts. It's saying two things: "this is what I am," and "this is how you should feel about me." Changing your garb is the easiest way to accomplish a change of character.
As a general rule, evaluation of who a character probably is occurs within at most five seconds of seeing them, so make that time count.
Change your mannerisms. I list this second because this is much harder to accomplish, though it's almost equally important as item 1. (The "almost" is because, in the chaos of a LARP, you frequently won't be able to tell what somebody's mannerisms are like until after you've made a snap-judgment.)
The explanation for this is basically a reiteration of costuming. Are you behaving like your life is petty, and can be sacrificed? If so, you're probably a mook. Are you behaving like you value your life, and you're not just some king's man sent to die? You're probably not a mook - or if you are, you're a terrified mook.
(If possible,) play the polar opposite of your typecast. I place this last because this entirely depends upon the premise of the game you're playing, the number of people playing, and the flexibility in casting. However, if you can, this is the most effective way to invalidate others' preconceived typecasts for you.
The logic is simple: they're expecting you to be a given typecast, but you're completely different - something they haven't really seen from you before. This disconnects the character you're playing from your explicit physical appearance. Thus, your typecast is erased.
As an example, I know a person who almost always plays the quiet-wizard stereotype. He's okay with that, but for the purpose of discussion let's suppose he wanted to change. He could pick the defining aspects of his character: magic, quiet, somber, sullen - and reverse them. He might play a disease-stricken villager, or a cheerful and friendly scientist, or... et cetera. It might be weird for you, too, to play something like this, but that's okay. You'll get used to it, and get better at it, as time goes on.
In your specific case, I would suggest finding a character that is completely nonviolent - they have, in their lifetime, had no reason to walk around armed, and had little reason to learn how to use a weapon or train for combat. That's a polar opposite of your existing typecast.
There's an important bit in here that I didn't mention, and you may want to consider this. It's not something I can support with research, but it's a philosophy of LARP that's served my group well over the past years. It can be summarized like this: who you are is not your physical body. This is signaled in dozens of ways throughout the game - through costuming, mannerisms, game mechanics, death mechanics, etc.
The reason we have this philosophy is because it's important that, when out-of-game, you're not conflating the person you saw in-game with the person you saw out-of-game. Once the environment goes away, everyone is back to the way they were. If others are tying your character to your physical body, the assumptions they make about your character may come up when you're you, not when you're in-game, and that's a problem.
The side-along consequence of maintaining this philosophy is that you're much harder to typecast, because the characters you play aren't defined by your physical form. To rephrase the above: your physical body flavors your character, not the other way around.
Much of this comes down to practice teaching each other that who you are is different than the characters you play. This is abstract, I know; but as a guiding principle, I hope it helps in the future. This is a side point, though; the body of my advice is above.