If the assembly was imported as an assembly, then it should be possible to delete the part.

If the assembly was imported as a part, then it's a collection of surfaces that made up the individual part surfaces, so there is no part to delete.

In the past a function called "Data Doctor" could be used to edit the imported information and delete unwanted geometry, including curves, points, and surfaces. It's not a lot of fun, but the unwanted geometry can be removed that way. I don't know if it has that same name, but the functionality should still be there.

Thank you. The model hasn't been imported into Creo, but the engineer created the whole model in Creo.
Could there be a reason why a part can't be deleted?

Not a STEP file then? Anyway, there are a few dozen ways to screw people over in Creo models - ask the engineer what he did that makes it "impossible."

I've never come across any impossible situations; I have come across people who build models in such a way that minor modifications take far more time than they should take.

I used to have a list of the top 100 ways to ruin people's day, but they individually take too much to explain them all and it does no good unless you have the experience to have made your own list, in which case you would not be asking. It took more than a decade of dealing with bad and misguided user models to get there.

When exporting from Creo all you have to do is hide a part and it won't be part of the export file.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

The model tree should either list a series of parts or surface bodies. Click through the list and delete as appropriate. There's no way to make this an impossible task, only a painful one if your extra "part" is a million tiny surfaces scattered throughout the tree.