This may be a little harsh but in the past with DSS ages 3 and 6 we give three chances to clean it up and explain that if they haven't done it after being asked nicely three times me or DP will tidy it up but the toy goes away in the cupboard. The threat of taking a toy away usually works and on the occasions we have packed up ourselves and put the toys away the boys have been less than impressed when they go to play with that toy again and it's in the cupboard.