Search Heads can be removed / shut off without any technical concerns. The biggest concern would be any knowledge objects ( dashboards, alerts, saved searches, lookups, etc) that are actively used on those search heads. If you don't need to use these, just shut the instance down.
If these are Search Peers also, meaning you have indexed data on them and actively search that data, you need to follow the remove a search peer instructions : https://docs.splunk.com/Documentation/Splunk/8.0.1/DistSearch/Removeasearchpeer
