The waste ink is a set of sponges inside the ink jet that absorb ink during the printhead cleaning.
For now you can keep printing by pressing and holding the printer's On/Off button for one second. Then wait 10 seconds and press the On/Off button again. Wait until the printer finishes cycling before trying to print again. See Canon's support page for details: http://www.usa.canon.com/cusa/support/consumer/printers_multifunction/multipass_series/multipass_mp700#ServiceAndSupport
. Search for Waste Ink Full.
When the waste ink tank is full, then you will not be able to print until the unit is serviced. Canon will service this for you. (Free except for shipping if the printer is under warranty, standard $$ otherwise) However under normal use, the waste ink full counter should be reached its limit near the expected end of life of the printer. Additional failures may occur soon.
This is a messy job and may be a lot more work than you want to do. Have plenty of newspaper to absorb the ink if you try this. You need to disassemble the printer to reach them. One set is under the area where the printhead sits when the printer is idle. The other is further down in the center. You will need to remove a few small screws to access the bottom set. Once you get them out, blot off most of the ink and then run under water until the ink is completely out of the pads. Blot the areas where the pads sit of the excess ink as well. Then dry the pads (you can use a blow dryer to speed this step). Then reassemble the printer. (Alternatively, order replacement pads from Canon)
for additional comments from others who have tried this.
Next you can reset the counter. (Do not do this reset before cleaning or replacing the ink pads; waste ink will smear everywhere.)
Resetting the Waste Ink Volume Counter
1) Enter SERVICE MODE.---
By pressing [Menu] then [Copy] then [#]
2) Select TEST MODE. (using + or - keys)
3) Press the [OK] key.
3) Press the  key. (TEST MODE)
4) Press the  key.
5) Press the  key. (INK COUNT).
6) Press the [OK] key.
7) Press the [Stop/Reset] key
9) Press the [ON/OFF]
I hope this helps.