"Wall up" is an idiomatic phrase that describes the act of building a wall or barrier to block or close off an entrance or area. There are many synonyms that can be used to replace "wall up," such as barricade, block off, close off, seal off, fence, enclose, contain, or isolate. Depending on the context in which the phrase is used, different synonyms may be more appropriate. For example, "barricade" implies a sense of urgency or danger, while "enclose" suggests a more deliberate or intentional action. Overall, using a variety of synonyms can help to bring a more diverse and nuanced vocabulary to your writing or speech.