Hi Kebab! Sounds like you might have aphids on your roses
- here's an organic website
that has some remedies for it
In the meantime, as a temporary solution, you can spray the bush with a hose - give it a little blast of water to knock the ants/aphids off the leaves. (Ants will herd the aphids for their honeydew...so that's why I'm guessing they're aphids ya have)
As for trimming it back - we have about 8 rosebushes we've been battling blackspot all summer long off and on. We had to cut some of them completely down to the ground in early summer and watched them grow right back. Don't be afraid to cut back a rosebush - my mom always said the more ya cut, the better they grow - and it's true! As a rule of thumb, when you deadhead a rosebush (after the flower
fades) cut it just above the first group of 5 leaves from the top of the stem. If you need to you can go all the way down to the last set of 5 leaves and it'll come back - but if ants/aphids are your only problem, I would try getting rid of them without cutting beyond general pruning - cutting won't necessarily keep them off your other good leaves!
Hope that helps!