How can a plug-in accidentally get used?
 
Since they are installed as part of the SONAR installer, you have a few removal options. Delete them. Disable them in Plug-in manager. Use a custom plug-in layout. Move them out of the VST scan path like was mentioned above. Do a clean re-install of X3, using the advanced install option, do not install them.

John that is a very strange statement. 

I sometimes accidentally apply a low pass filter @ 100Hz across the master bus. It takes me ages to rebuild the whole project from scratch. I wish there was some kind of safeguard in place to stop me doing this. 

HAH, did that once when I was testing some new monitors. Accidentally saved the test project with a high pass filter on the master. Took a few minutes the next day to figure out why the monitors were so weak.

With "accidentally" I mean the following:
 
Anything is OK if it sounds fine and everyone are happy. Sometimes, depending on the work you do in a studio, there has to be some "standards" etc. If everyone invents new things all the time and send projects here and there etc. it can lead to confusion and problems. Let's say someone thinks a Nomad de-esser is fine and we already have the Fabfilters De-esser and use them in a certain way and our client has the FF's too etc. then you may chose to "prevent" someone from "accidentally" using Nomads de-esser...
 
Nothing strange about it. Just being practical and trying to avoid confusion and mistakes, that's all.
 
I didn't mean to a situation like if someone was leaning over to the right to grab a cup of coffee and then accidentally enabled the Nomad de-esser on track 18... ;)
 
Thanks for your replies...
Thomas

Skyline_UK
 I thought 'old' was cool......

They're "vintage". FWIW - I like them.

I'm old and I'm cool!

These troll threads are getting tiresome.  Nobody accidentally uses a plugin.  Ampfixer is spot on and I bet most of us could guess within 5 tries which member this is.

I think in defence of the OP he made a reasonable explanation just a few posts earlier.  Maybe it's the choice of word "accident" that is confusing, but what I believe he is referring to is maintaining a standard set of plugins in a studio environment that are consistent with what a client is using.  That way they can safely exchange project files without either party dealing with a "missing plug-in" issue.  
 
Of course, one doesn't literally "accidentally" use the new plug, because clearly there is some intent in selecting it.  But if you had many people working at a studio then you might want to avoid the situation of having someone select a plug-in by removing it completely.  I get that.

Oh boy. Locking this this one.