Clindamycin is a poor choice for treating UTIs. Please consider taking her to the vet to get proper medication for it. It is true that they very commonly are inflammatory vs infectious... in which case antibiotics won't do much good as she would need to be put on a proper antiinflammatory medication to treat it and a proper antibiotic (if needed).

There are diets out there to prevent crystal and stone formation, but you would need to get it from your vet as it is a prescription diet. The main thing you want to look for is one that has a proper pH to avoid the formation of those specific crystals/stones. And different crystals/stones form in different pHs, so you'd need to know which kind you were dealing with to know what diets you'd need to look at.

Forgot to add... one thing that can help prevent (but isn't 100%) is offering wet food to help increase her water intake in attempt to dilute urine and make her urinate more often so urine isn't sitting in her bladder for as long to hopefully help prevent crystals/stones from forming.