Lock a table in Sybase ASE 15.5
I think you should look at locking mechanism of table, there are three types of them - AllPages, Datapages and Datrows, you can read more about them at:

Locking Mechanism in Sybase

If you alter locking of table, update/delete and select can take place together without any blocking on mentioned table.

Inside query, you can put noholdlock on a table or other options for other users to access/no access based on these options as mentioned in below link:

Controlling Isolation

In order to answer your question in detailed level, we need to have more clarity on question. Your question doesn't explain details of issue. I hope above helps.