How to stop decrypting data after opening master key in SQL

I have implemented data encryption using SQL Server symmetric key encryption technique using a certificate, master key with password.

When we backup database from PROD server and restore to a different server, we open the master key using OPEN MASTER KEY DECRYPTION BY PASSWORD = ''... statement.

Password for master key is being managed by a SQL admin.

After opening the master key, we are able to decrypt the column data using DecryptByKey function. However, once we have done some analysis, we want to stop decrypting the data. No developer should be able to decrypt the data after a while.

How to stop decrypting the data or close the master key so that no developer can see data in plain-text?