I would transplant it to a new area that wasn't affected by the poison; or dig it up, replace surrounding soil with new clean soil. If that's not possible, flush the surrounding soil with water to leach out as much poison residue as possible. Yes, trimming off the flower buds is a good idea. Let it regrow roots & foliage first.

I'm wondering how they got poisoned, and how you knew they'd been poisoned? If they were poisoned, do you know what with? If the 'poison' was a herbicide, some sit in the soil for a long time, some for a short time, some not at all, so its hard to know quite what's happened. Did your lawn recover well, if it was affected in the same way?

Assuming the grass did recover, if the roses are not ramblers, did you prune them when they should have been done, in March? Its a little late to hard prune them now, but if they look truly awful, then you can, but it depends what varieties of rose they are with regard to flowering again this year. Certainly, a good feed, particularly with a specialist rose food such as Toprose, would be a good idea, along with mulching round the base with composted animal manure (without raising the soil level around the mainstem).

What was the size of the total affected area? Was the lawn a complete kill?

Humates, olmates, and fulvates all help with this. The first two work to de-tox the soil, when applied fairly heavily and watered well in. Fulvates can work within the plant, when applied as a dilute foliar spray.
Sources of these vary: mixtures of these, usually derived from carbonaceous shale, or compost tea, are sold in liquid or solid form as "humic acid" or "humates"; well made compost tea, itself, is less easy to make or apply, but has the benefit of containing living fungi and bacteria, many of which will eat the toxins, once the toxins are combined with the humate complexes.