Jewish law forbids the defendant to refuse. It is a violation of Torah law, similar to eating non-kosher food or violating other Jewish laws.
In the event that there is an existing agreement to come to a particular Beth Din (for example, if the parties signed a contract which refers any disputes to a particular Beth Din), the Beth Din can hold a Din Torah in the absence of the person who was summoned. The Beth Din considers the evidence that the claimant brings and issues a ruling on that basis. Otherwise, the summoning Beth Din generally issues a “heter arkaos,” which grants permission to the plaintiff to go to secular court, so the defendant does not simply avoid a hearing.
A summoning Beth Din may issue a “seruv,” or a contempt order. A seruv is simply a public declaration by a Beth Din that someone was summoned to Beth Din but refused to meet their obligation under Jewish law to appear in front of the Beth Din. Sometimes, Jewish communities or synagogues impose sanctions on such people, such as not giving them aliyos (being called up to the reading of the Torah) or refraining from social interaction, to pressure the person to meet their obligation. The Beth Din is not necessarily involved at that point – the community or synagogue decides what measures are appropriate.
Occasionally a summoning Beth Din may issue an “ikul,” a restraining order.