If you take freq antibiotics, does that kill dental bacteria and stop cavities?

No. Taking antibiotics long term is not good for you. Antibiotics will reduce the numbers of "good" bacteria in your body as well as the bad. Why are you taking frequent antibiotics? Antibiotics will not kill plaque bacteria streptococcus mutans in your mouth. The only way to stop dental decay is through frequent brushing and flossing and seeing the dentist at least every 6 months.
No. These bacteria are part of the normal bacterial population in the mouth. Regular brushing, use of bactericidal mouthwash, regular dental inspections will help, but taking antibiotics carries a high risk of multiple other complications without being likely to affect the rate of cavity formation.