Is there a control that will stop my Weil McClain from cycling at night to heat hot water?

Yes, probably. Assuming you have a domestic coil in your boiler, and not an indirect hot water maker, you could rig a time clock to interrupt the aquastat circuit , which is maintaining tank temperature. I doubt there would be a huge savings though. If it comes on a few times during the night and only runs for a minute or two, to keep the set temperature, it would probably come on at the end of the timer cycle, in the morning, but have to run for a longer period of time to make up for the greater reduced temperature in the tank.

I guess what I'm worried about is that last summer I calculated that I used about a gallon a day of oil for hot water. That's going to translate into over $100 a month with oil prices where they are. I'm wondering if getting an independent hot water heater will be more cost effective.

What temp is the aquastat set to now? Maybe it's too high and you need to lower it.

A gallon a day, your figures or mine are way off Ng might cost you

I'm in the same boat as you. For the last two years I've contemplated installing a 30 gal electric water heater for the summer. I just don't know that there would be a real savings, especially now I hear electricity prices are going to go up due to some new taxation, environmental bill in congress. As Mikepier said, I do turn down the aquastat temp during the summer

Not sure what you mean with Ng? I'd love to pay $10 a month for hot water where do I sign up?

Natural Gas