This is a bit of a tricky question, because technically
MakeCode is not what causing the rest of the code to be analyzed as code. A hint for that can be the fact that
MakeCode actually returns the length of the current instruction. It is only later that additional following bytes are being converted to code (if possible).
Thing is, that for IDA to do what IDA is good at (that is, successfully disassembling large code bases) it has it's own background code analyzer. That Code Analyzer is in charge of orchestrating high level disassembly operations such as figuring out where functions start, make sure all functions are analyzed, handle overlapping code to the best of it's ability, etc...
One of the features of IDA's Code Analyzer is the code analysis stack. That stack contains "leads" to additional unexplored code and hints towards where additional assembly code might be found. While
idc.MakeCode indeed only converts a single instruction to code, it registers the end of that instruction with the Code Analyzer, which then proceeds to analyze and convert the rest of the binary to code (as long as valid instructions are found).
To disable the auto analysis running in the background hit the "Options" menu and the "General" function in it. In the dialog select the "Analysis" tab and uncheck the checkbox labeled "Enabled" to the left, as shown in the following picture
Fun fact: You can call
idaapi.analyzer_options() to show that dialog. You'll still need to uncheck the checkbox and hit the "Ok" button. Unfortunately it seems there's no API for that.