How to decline a potential PhD supervisor?
I wouldn't say you 'wasted' their time. All supervisors are well aware that most applicants apply to more than one institution at a time; wisely so, especially that PhD admissions for good universities are highly competitive, and one shouldn't risk their chances applying only for one place. 

Unless you made a promise or commitments to be working with them, you've been an applicant and have acted as such. As an applicant you have the option to decline an offer, so this is totally normal.

Therefore, you can contact the other supervisors; the sooner the better so that they can consider their other options too. You can thank them for their time and the interest they had in you, and explain your situation like you did here. They will likely understand the situation (TBH this should be happening a lot for them).