As with other sexually transmitted diseases, the way to prevent infection is to refrain from intercourse before marriage and then to have intercourse only with your spouse, who has intercourse only with you.
If you do have intercourse with someone else, use condoms. There is some indication that this might keep you from becoming infected with this particular sexually transmitted disease initially, but if you continue to change sexual partners, even though you are using barrier contraceptives, you will probably get a sexually transmitted disease. And remember, if the syphilis sore is on a part of the body not covered by a condom, there is no protection.