Something like a lock would be much better than what is currently available. You should start a product idea.
Global reference control is set through File-->Options-->Assembly.
Individual basis is set through File-->Prepare-->Model Properties-->Model Interfaces...you can also set it by right clicking in the model tree-->reference control. I hope it works for you. You might be able to create a subassembly of parts that you don't want external references for, etc. but something tells me that's more work than it's worth and may not get you what you want.