The KArlsruhe TRItium Neutrino (KATRIN) collaboration aims to determine the neutrino mass with a sensitivity of 0.2 eV/c2 (90% confidence level). This will be achieved by probing the end-point region of the β-electron spectrum of gaseous tritium with an electrostatic spectrometer. A gold-coated stainless steel disk defines the reference potential for high-precision neutrino mass measurement, and it terminates the β-electron flux as the physical boundary of the tritium source. This so-called rear wall is exposed to tritium, which leads to adsorption and absorption. This in turn leads to systematic uncertainties for the neutrino mass measurements that need to be understood and mitigated. In maintenance phases, during which the gaseous tritium source was emptied (<10−5 of nominal gas density), the activity that accumulated on the rear wall during normal operation was monitored using beta-induced X-ray spectrometry (BIXS) and direct observation of emitted β electrons with a silicon detector. Dependency of the observed activity increase on the integral tritium throughput was investigated and found to converge from limited exponential growth to continuous linear growth. This paper gives an overview of the results that were obtained using several methods of in situ decontamination of the rear wall while continuously monitoring the activity. The decontamination methods included heating during continuous evacuation; flushing the system with nitrogen, deuterium, or air with residual humidity at different pressures; and illumination of the rear wall with ultraviolet (UV) light. These well-known methods led to only a small (15%) decrease in the observed activity. However, a decrease of the surface activity by three orders of magnitude in less than 1 week was achieved by combination of different methods using UV light, a heated surface, and a low (5 to 100 mbar) pressure of air inside the chamber, leading to the production of highly reactive ozone. This proved to be by far the most efficient method, drastically reducing the contribution of the rear wall surface activity to the β spectrum of the gaseous source.