Do cell walls prevent cancer?
Plants do have uncontrolled tumorous growth. Though in almost all known cases, it is pathogen induced; such as in the case of the bacterium- Agrobacterium tumefaciens. Viruses can also induce plant tumors. Theoretically, the cell wall will pose a mechanical barrier to cell division. Also, cell wall synthesis is an extra biochemical step during cell division. Surely, the presence of a cell wall will disallow any metastasis if it might occur. There is an old article which says that cell wall structure differs between the normal plant tissue and the crown galls (tumor). However, this is with respect to Agrobacterium adsorption sites.

I am not aware of any natural plant tumor or the role of telomerase in plant tumorigenesis. However, there is one paper that says that telomerase dysfunction leads to disorganized growth.