You could try some mushrooms or you can buy those little packages of mozzarella and mushroom breaded pieces and try putting them in your spaghetti or you could just use really hearty tomatos. You could even use the replacement kind but those don't always taste so well.
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I realize that this is slightly missing the point of this question and this is not intended as an insult or anything, but have you considered moving on from meatballs and pasta? If you are unable to find a good substitute for meatballs after all the answers have come in, you might want to consider discovering another new type of pasta. I personally have seen tofu balls with meat flavoring as well as attempts at vegetable meat balls. In all cases, I haven't found anything that is as good as regular meat.

Many of the vegetarians and vegans that I know have mentioned how their dietary changes have led to new doors being opened in their lives. Perhaps now that you have moved on from meat, you can move on and find a more suitable dish. Pasta cooks are continually pushing the boundaries and a large amount of pasta dishes are vegetarian friendly. Try something new. You might find something that you like even more. I'm not a vegetarian but I have seen a number of good no-meat pasta recipes in recent years.

These are just my feelings on the subject.
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soy meat balls.
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Tofu is a very versatile product. If you freeze it, it changes its consistecy, and becomes almost like mince. You can combine it with other ingredients, such as spices, chillies or tomatoes, cook it, and enjoy! Alternatively, if there is such thing as soy meatballs, give those a try.Good Luck!
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I am a former chef and eat alot of vegetarian meals becauce of my medical condition, and I have found TVP ( Textured Vegetable Protein) is a good meat like substitute.

I chop my onions and add them with my olive oil in a hot pan, brown them and then add a cup of TVP and let that toast for 5-10 minutes, then add my tomato products and dried herbs, slow cook it for 30 minutes and season it, sometimes I add fresh basil at the end for zip.

You can make a bolognese sauce, with celery, carrots and made the same way, by adding soya milk or a bit of evaporated. One thing is it will give a subtle meat like mouth feel, but is bit more headier, but will not spoil due to the meat in most sauces is the problem. I use it in chili, stuffed peppers, cabbage rolls and even mexican dishes like tacos and burritos.
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http://www.yvesveggie.com
Of course it's expensive, but it's good.
You can make your own, but that's time and time is money.
Mushrooms are a good sub as they are pretty meaty. The Taiwanese make great meat subs from mushrooms inc. meat balls.
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Soy crumbles; morningstar farms, boca, or lightlife.
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My family and I are ovo-lacto vegeatarians and we use the Shnieders(dont know if I spelled the name correctly) veggie meatballs. We love them, even my close minded mother when it comes to vegeatarian foods loves them. If you look on vegetarian web sites you may find some grocery stores in your area if not your local grocery store should have a vegetarian section. Look in the freezer area as well. Hope this info helps. Remember to make sure you’re getting all you need (iron, protein, B12 etc.)