Is it possible to comment out a circuit in LTSpice?
There's no simple answer other than manual disconnectioin.  Even this can be tricky depending on what you're simulating.  This is one of mine:-

The random voltage source (B1) has been disconnected and a sine wave (V4) applied.  This worked for me as all I was interested in was the output signal.  Unless you're simulating a Xeon processor out of discrete transistors, I've never had a time problem just allowing the computer to simulate lots of components, even if not required.  You can always have a gin whilst waiting.
The problem arises though if you're looking at things like power consumption.  Clearly Power will still be consumed if you disconnect an unwanted signal from an unwanted module.  So you disconnect it at the power side.  That might then leave the output end (with all its impedance s and conduction paths) possibly affecting your wanted circuit.  This might mess up a gain sweep.  So disconnect it at both ends?  And if there are other connections?  And if there are multiple optional modules?
I don't think that it can be done in more complex cases than my example.  Some one mentioned  inserting a high value resistor.  This is akin to circuit disconnection and brings with it all the problems I outlined above.  Even if you set loads of resistors in a parameter list, you'd probably have to spend more time getting that correct than just amending the circuit whole scale.
Thought.  Do you always need to disconnect?  There are parameter lists, and even the ability to do a Monte Carlo simulation (if you really can't figure out component values).  A parameterised list might be suitable in some cases, but there will be some that you just have to redraw.