ant powder or sprays. You can get cheap powder or sorays from poundland, whch work as well as the more expensie brands.

The spray will kill any ants it is sprayed on and the powder will stcik to any ants that walk through it and will be taken back to the nest.

The plant reference under your question is Verbena bonariensis - if you're implying you have ants all over your plants, spray them for aphids - ants don't harm your plants at all, they only use them to get at the honeydew which aphids secrete. For ants elsewhere, follow Alextb's advice above.

Generally speaking ants are very useful in a garden, helping to break down waste and encouraging beneficial fungi, and form an extremely important part of the ecosystem as a whole, so I'm not an advocate of killing them off unless absolutely necessary.

Indiscriminate insecticides (such as ant powders/sprays) are incompatible with wildlife gardening and will kill off a wide array of beneficial insects....like ground-nesting bees for instance Alex.... as well as harming the predators (birds, amphibians, mammals, predatory insects etc.) that feed on them.

That said, ants they can become problematic in pots or disruptive if they build nests in the wrong place such as in pots or a lawn for instance.

Ecocharlie sell something called "No Ants":
http://www.ecocharlie.co.uk/grow-your-own/pest-and-disease-control/nemasys-no-ants.html

Which is an environmentally friendly nematode solution that you simply water into the soil and which will effectively drive the ants out of the lawn and the immediate area surrounding it.

As Bamboo says, if your ants are simply on one individual plant or group of plants then dealing with the aphid issue will cause the ants to leave.