To delete the UM logs try:
:local Flag
:set Flag [/tool user-manager log find]
:if ([:len $Flag] > 0) do={/tool user-manager log remove [/tool user-manager log find];}
To export to file then delete UM logs try:
:local Flag
:set Flag [/tool user-manager log find]
:if ([:len $Flag] > 0) do={/tool user-manager log print append file log;/tool user-manager log remove [/tool user-manager log find];}