Can iphone messages be blocked by keywords?

No, but you can intercept the specified number. Take the iphone7 as an example, the method is as follows:

1. Open the settings and select the information option.

2. Turn off the iMessage option inside, as shown below.

3. Open the following options for filtering unknown senders, as shown below.

4. Open the information and select the information to be blocked.

5. Select the point below the upper head as shown in the figure below.

6. Select the information option, as shown below.

7. Select an avatar, as shown below.

8. Select the number to block the call, as shown below.