I think we can help you better if we have the bigger picture of your issue. Some of these questions might seem off target, but there may be an underlying issue which is causing the problem.
Where do you live?
What kind of grass do you have?
Do you know what kind of ants you have?
Are they causing any obvious issues...other than the mounds being unsightly?
What poison did you use?
How often do you water and how long?
How high/low do you mow?
When did you fertilize last and what did you use?
Is there much shade where the ants are?

The reason I asked these questions is most people don't have ant problems. You might be doing something different from what most people do that is bringing the ants to your lawn. Or you might be using the wrong ant killer for your type of ants. Different ants feed on different materials and will avoid everything except the food they are looking for.

Where do you live?
Dallas area

What kind of grass do you have?
Bermuda

Do you know what kind of ants you have?
I'm not sure exactly which species they are. I assume they're common fire ants. Dark red/brownish color with darker or even black abdomens. (I've attached a picture if you would know more)

Are they causing any obvious issues...other than the mounds being unsightly?
Nothing that I'm aware of. Just mounds

What poison did you use?
For mound treatments, I've used 'Spectracide's Fire Ant Killer'. And for the full yard treatment, I'm pretty sure it was OrthoMax

How often do you water and how long?
Twice a week for about 8 mins a zone

How high/low do you mow?
2 1/2"

When did you fertilize last and what did you use?
During the summer. Scott's fertilizer, the one for summer protection

Is there much shade where the ants are?
Most that I've seen are in full sun areas

To me they appear to be what I call fire ants. We have had success with the direct application poisons like Amdro, or similar poison. If one does not work, try another, as some are baits and other are all poison.

You must continue the process as they appear, it is not a one time situation. These ant hills are deep, and some time quite large, so a one time application may not kill the entire colony.

We have been working on them about two years now, and we still get the mounds. I bought a dozen of those little orange flags on a wire like they used to mark water, TV and electrical line prior to any construction projects. I carry them on my riding mower. After mowing across the mound, I place a flag into it. Later we come back and apply the poisons