There are so very many methods of banishing. Unless a pentagram helps you feel it is more effective, or it is specifically part of the tradition whose prescribed methods you are using to banish, then no: It won't make the effort any more effective.
Are you trying to clear a space (like remove a spirit from your home), from a person, or are you looking for something a bit different?
I suggest researching the LBRPaka the Lesser Banishing Ritual of the Pentagram.This ceremonial magic ritualis used by a lot of people however it was devised and used by the original order of the Golden Dawn.
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There are indeed several banishing methods available, and I agree that the lesser banishing is a very good one if you want a formal and ritualised process.
There are four main keys to being effective in a banishing, that you want to seriously consider before going to proverbial war though.
First, what is your own practice, and where are you within it. This will guide you on what symbols and processes will resonate most strongly with you. Think of it as making sure you are selecting the weapons you know. (As they say, Know thy self)
Second, make sure you have some understanding of the being you are evicting. For lesser events it may just take a cleansing and making a proper threshold. For others it may take more specific efforts. For more potent beings you will want to be able to specifically address them and call them out/challenge them. Or to make offerings of peace and bargain with them to leave. (In effect, know thy enemy)
The third and fourth points are tied together. And are effectively lynchpins to the previous two;
How experienced are you, and what authority have you developed. Be careful on guaging yourself and how far you are within your practice. Just like with people, if you intend to start a fight you'd better be prepared for them to fight back. If you aren't, then you risk stirring up the pot and just make things worse. Better to be honest with yourself, and get direct help from someone more knowledgeable if you can. That way you can follow their lead, learn, and become better yourself.
Authority is incredibly important, and only comes with experience. It is not only the abity to give command, but also to have focus, confidence, and discipline. Authority over the Self is vital as it is what protects you from reprisals.
My advice is when you choose a course of action, don't just research the rite you intend to perform. Also research how to prepare, cleanse, and focus yourself before and to ground, release, and shield after.
If you have more questions on specifics, don't hesitate to post them up in the thread here.
You have received some excellent advice from others here on how to go about banishing unwanted entities and I don't have anything to add to what has already been said.
Something else to keep in mind is that banishing is just the first step in ridding yourself or where you live of evil spirits. Once you have successfully banished them, then you need to ward your living space and create personal shields for yourself so that they don't just move right back in.
There are any number of ways to go about doing this, but here are a couple to start you out:
The usage of a Pentagram depends on its representation to you and your path. It could help you build concentration and gather sufficient energy for your banishing work, and possibly have other benefits according to the meaning it has in your personal occult path.
Thus, I would strongly advice you to have an overall understanding of the meaning of each symbol or element you use in your craft. This way you can utilize energy more effectively and there's less room for mistakes.
That being said, using a pentagram even if it's not a part of some sort of path you've taken wouldn't necessarily be useless. The belief in its usefulness itself could have a positive effect on your work or even create an egregore that will aid in the banishing.
