"用立法规定" can be translated into English as "stipulate by legislation" or "regulate through legislation".
Here are some example sentences to illustrate the usage:
1、 We should stipulate clear penalties for environmental violations by legislation.
(我们应当通过立法规定对环境违法行为的明确处罚。)
2、 The government plans to regulate the use of personal data through legislation to protect privacy.
(政府计划通过立法规定个人数据的使用,以保护隐私。)