Try Bayes’ theorem. Ask them to give percent likelihoods for the following:

A. The odds that the government (or whoever) is trying to kill everyone, before taking the evidence of excess deaths into account
B. The odds of seeing excess deaths for any possible reason, not just their conspiracy hypothesis
C. The odds of seeing excess deaths if the conspiracy hypothesis were true.

Then logically, the odds of the conspiracy being real given the excess deaths should be A*C/B. If you disagree on the outcome, you must disagree on one or more of the assumptions (probably A—if it’s B, you can find the objective odds by checking historical data).

If you still disagree on the prior assumption (A), you can set aside the excess deaths argument and ask what other evidence led them to form that prior assumption. Then you can repeat the process until you either reach agreement or they’re left with an assumption with no evidence.