From MikeS’s cite:
I don’t think that this has been completely answered. A positron has the same charge as a proton, but they’re different in other ways. A positron is opposite to an electron in every way except mass, not just in charge. A proton and an electron can’t annihilate, because there are other properties like baryon number and lepton number that don’t balance out between them. In fact, the lowest-mass combination of particles you can get with those values of total baryon number, lepton number, and charge is… An electron and a proton. So you can’t combine an electron and a proton to produce any other particles, unless you add more energy to the system.