杜 is a bound morpheme (b.f.) in modern Chinese, meaning it rarely stands alone as a word. Its most common uses are as a surname (Du) and in compound words like 杜绝 (dùjué, 'to put a stop to, to eradicate') and 杜鹃 (dùjuān, 'cuckoo; azalea'). As a verb root, it carries the literary sense of 'to block, to stop', but you won't use it by itself in everyday speech.