It may help, but if it's hot by you and they're getting afternoon sun, it may not do any good. I have the 'Moon' series. They can handle the heat better as long as they only get morning sun (mine get about 3-4 hrs) and don't dry out. I also mulch them. Mine have done well all summer even when it's in the 90's for quite awhile.
If you cut them back, just keep the watered well. Btw, do you know which kind you have? Let us know how they do if you cut them back.
Good luck!

Kat

I'm not sure what kind I have they are the deep blue ones, and this year has been the best I have ever done with them, they are still so pretty and filled in a patio bed nicely. This is the first time I had them in the ground, I usually put them in pots, butI did what the other poster said, they get am sun and plenty of water. What surprises me is they are the yuckiest dirt, I can't even call it soil, and they are still pretty as can be living with red a deep purple petunias, creeping thyme and yellow marigolds.

You should be able to cut them back some, I wouldn't do a whole lot, and water well. My alyssum benefites from a good haircut once a month, comes back nice and full of blooms.

Ok, I did it, I whacked it back. Only time will tell, I guess. LOL! Thanks for the responses.

I forgot to mention that I have mine in the ground also. I tried them in pots, but they didn't like the soil getting dry. At least in the ground you can mulch them and that soil holds the water better than hanging pots.
Herbivore, just remember to not let them dry out. Are yours in pots or the ground? In about a week you could give them some fertilizer. Let them recover a little from their 'haircut'. LOL!
Maidinmontana, that sounds like a really good combination you have. I have mine in a raised bed, with yellow and deep red ever blooming begonias, white aslyssum and a purple leaf heuchera. They are starting to hang over the edge now and I just love that. These blue lobelia are one of my favorite annuals that I have to have every year. :)

Kat

I find that the lobelia I buy in the spring poops out by mid July. I have never sheared it back but really the plants look like they are finished for me by then. I have two secrets to having them around throughout the season. One is avoid the eyecatching plants that are in full bloom when you buy them in the spring. Get the small starters in 6 packs. They are much cheaper and will last longer. My other secret is to grow some from seed in the spring. I plant some indoors and wintersow others. The winter sown ones are actually done in early April. The ones I start from seed are just getting it together when the other ones poop out and typically survive the serious heat of summer and flower their heads off until they are killed by the frost. I grow some in pots and others in the garden where they do well flopping over rock walls.