This setting is active immediatly

With flamerobin (Database Properties) or gfix
http://www.destructor.de/firebird/gfix.htm
Enable Forced Writes

gfix -user SYSDBA -password masterkey dbserver:/db/mydb.fdb -write sync

Disable Forced Writes

gfix -user SYSDBA -password masterkey dbserver:/db/mydb.fdb -write async

Regards