I'm assuming you are drawing single line in plan view.
In this situation I have found that most of the time I can hide a reducer by rotating it 90 degrees.
Select the reducer, and then click the "rotate" symbol that appears next to it. Since the reducer symbol is 2D it will now appear to be just a line.
Do you want to still see the flange tick mark on the tee, or other fitting? If you do, the best thing to do is use a reducing tee or cross and tell each side of the fitting to be the pipe diameter. Not all families will do that... you have to use the reducing version, which does not typically insert itself when you have that connection to make. Let it do whatever connection it wants, then swap it out with the other family... load it into the project, and click on it and you should see dimensions on each connector.... just click the dimension and set it to the pipe width.
If you want to blow away all pipe tick marks at connections, whether they are reducers, elbows, etc... so you only see single line representation for all pipe... just go into the family and fix it... DON'T DELETE the tick marks for the flanges!!! Just select them, go to your properties ( think element properties ) and turn them to NOT VISIBLE so you can't see them in the model in plan view.
That'll be $10... ; )
BJ