There are different ways that a .prt can be attached to a drawing file. Here are a few of the most common.
The most direct way is being a drawing model. In the drawing select model/rep icon above the model tree to see if it is there.
If it is there, you will need to remove it from the drawing by selecting Drawing Models from the layout tab then select Del Model from the menu and select the model from the list.
The part can also be part of an assembly on the drawing
There can also be an annotation left over from the part. Select the annotation tab, expand all of the annotations and look for a suppressed annotation and delete any found. They will have a black square before the name.
There is always more to learn in Creo.