Effect of stopping 1 gram of penicillin v after a day upon realising you don't need it as have no symptoms except tonsil stone?

Penicillin & Strep: If you were diagnosed with Strep throat, it is important to take full course of Penicillin to prevent the rare complications of step which include rheumatic fever which effects your heart and a kidney condition called glomerulonephritis

None: If you do not need an antibiotic for a bacterial infection then stopping it at any time should not cause any problems.

Shouldn't stop: This medication is used for strep throat and likely why it was given to you. As you point out a tonsil stone requires no treatment. However if you had a strep test that was positive, antibiotics are needed to lessen the chance of you getting rheumatic fever and subsequent rheumatic heart disease. Incidentally antibiotics may lessen birth control pill effectiveness, so plan accordingly.