The main issue when speaker B spills on speaker A microphone (be it a lapel or not) is the relative level between A and B on A's mic.
By reducing the sensitivity of the transmitter, you are not changing this ratio, you are probably just under modulating the radio link (which is not good!). Don't reduce transmitter sensitivity to solve a spilling issue.
Causes for spilling
Distance between the speakers. The closer they are, the more spilling
Directivity of the microphones. Most lapels are omni directional. But
there are cardioid lapels. The more directivity, the less spilling
Relative acoustic level of speakers. The more loud speaker B is
compared to speaker A, the more spilling from B into A you get.
How to deal with spilling
Depending on your hardware:
Use expanders/gates on each microphone so that the spilling level is just under the threshold. Properly setting such a dynamics processing require experience.
If you are recording each mic on it's own track, record as it is (with spilling) and then mix appropriately in post. It means riding each mic fader up or down depending on which speaker is speaking at a given time. (Or use the above expander/gate setting in post).
If you are live mixing several microphones (recording the show on one track), you will have to live mix the mics. This requires experience, as you must, best as possible, anticipate next speaker to gently open it's microphone while closing the previous speaker's microphone.
What when several (or all) speakers speak at the same time ?
You compromise between reproducing the mess, or privileging one of the speakers (the anchor maybe). Editorial might ask you to close some mics, or leave you on your own.