Federal employment laws (29 CFR 785.27) generally forbid employers from requiring employees or potential employees from completing training without compensation. Employees who are furloughed, on maternity leave, sick leave, vacation or between on-call/temporary work assignments can wait until they return to work or be paid while they complete training.
As a supervisor, you may ignore notifications or reports for people who are temporarily off-work until they return. If the employee will be off for an extended time and the reminders are causing problems, you can usually remove tracks for job-safety related training. For other programs that are auto-assigned, you can reach out to the training program owner to see if the training can be waived temporarily. The program owners will decide based on their resources to maintain waiver lists.