First you can not use deadly force to stop someone from trespassing.
(notice in neither section is trespassing listed)
But you can use force to stop them, and make them leave.
For someone to commit trespassing you must "give notice". Notice means:
(A) oral or written communication by the owner or someone with apparent authority to act for the owner;
(B) fencing or other enclosure obviously designed to exclude intruders or to contain livestock;
(C) a sign or signs posted on the property or at the entrance to the building, reasonably likely to come to the attention of intruders, indicating that entry is forbidden;
(D) the placement of identifying purple paint marks on trees or posts on the property, provided that the marks are:
(i) vertical lines of not less than eight inches in length and not less than one inch in width;
(ii) placed so that the bottom of the mark is not less than three feet from the ground or more than five feet from the ground; and
(iii) placed at locations that are readily visible to any person approaching the property and no more than:
(a) 100 feet apart on forest land; or
(b) 1,000 feet apart on land other than forest land; or
(E) the visible presence on the property of a crop grown for human consumption that is under cultivation, in the process of being harvested, or marketable if harvested at the time of entry.
The problem is you can not arrest for criminal trespassing (yes, a citizen can make an arrest for a felony or breach of peace).
All you can do is call the police (which isn't going to solve much) or start being rude back. As soon as you see them tell them they are trespassing and you can use force to make them leave under Chapter 9.41 of the Texas Penal Code. If they refuse to leave then it is up to you if you want to use force or call the police.
Since other have mentioned the use of deadly force I will explain the law (there is a lot of misconception).
You can only THREATEN deadly force if deadly force is justifiable. So if you are legally able to pull a gun on someone you are also legally able to shoot them.
You can use deadly force if:
(a) A person is justified in using deadly force against another:
(1) if the actor would be justified in using force against the other under Section 9.31; and
(2) when and to the degree the actor reasonably believes the deadly force is immediately necessary:
(A) to protect the actor against the other's use or attempted use of unlawful deadly force; or
(B) to prevent the other's imminent commission of aggravated kidnapping, murder, sexual assault, aggravated sexual assault, robbery, or aggravated robbery.
Notice that it lists crimes for which you can use deadly force, so you do not always have to be in fear of your life.
So you can use deadly force to stop the above mentioned crimes. Notice trespassing is not listed. You can not use deadly force to stop trespassing. Go to the link below to look up these crimes because you must be sure that they are committing these crimes.
(b) The actor's belief under Subsection (a)(2) that the deadly force was immediately necessary as described by that subdivision is presumed to be reasonable if the actor:
(1) knew or had reason to believe that the person against whom the deadly force was used:
(A) unlawfully and with force entered, or was attempting to enter unlawfully and with force, the actor's occupied habitation, vehicle, or place of business or employment;
(B) unlawfully and with force removed, or was attempting to remove unlawfully and with force, the actor from the actor's habitation, vehicle, or place of business or employment;
What this is saying is it is presumed under that law that if someone breaks into your home, business, or vehicle that it is reasonably to use deadly force. you do not have to wait to see if they are a threat, you can assume that they are a threat.
(c) A person who has a right to be present at the location where the deadly force is used, who has not provoked the person against whom the deadly force is used, and who is not engaged in criminal activity at the time the deadly force is used is not required to retreat before using deadly force as described by this section.
You can not use deadly force if you are committing a crime or provoked the situation