This is not an ArcGIS issue, it is a SQL Server issue. That said, it isn't really a SQL Server issue either. SQL Server, like most DBMSes, will gobble up all the memory they can because the more a DBMS does in memory the quicker the response times.
If SQL Server is running on a dedicated host, which I strongly encourage, then let it do its thing and dynamically manage its memory usage. If SQL Server is on a shared host, or you feel compelled to limit its memory, there is a system wide setting to limit its memory usage.
In terms of "keeping the RAM down," don't, outside of possibly setting an overall limit. Microsoft and other DBMS vendors invest a lot of time and money into memory management, and the DBMS is constantly looking at all of the queries being performed and deciding what to keep in memory and what to page out. There are few people that can manage memory better than the DBMS itself.